[#42] Add contribution guide

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

CONTRIBUTING.md

@@ -0,0 +1,155 @@
+# Contribution guide
+
+First, thank you for contributing! We love and encourage pull requests from
+everyone. Please follow the guidelines:
+
+- Check the open [issues](https://github.com/nspcc-dev/neofs-rest-gw/issues) and
+  [pull requests](https://github.com/nspcc-dev/neofs-rest-gw/pulls) for existing
+  discussions.
+
+- Open an issue first, to discuss a new feature or enhancement.
+
+- Write tests, and make sure the test suite passes locally and on CI.
+
+- Open a pull request, and reference the relevant issue(s).
+
+- Make sure your commits are logically separated and have good comments
+  explaining the details of your change.
+
+- After receiving feedback, amend your commits or add new ones as
+  appropriate.
+
+- **Have fun!**
+
+## Development Workflow
+
+Start by forking the `neofs-rest-gw` repository, make changes in a branch and then
+send a pull request. We encourage pull requests to discuss code changes. Here
+are the steps in details:
+
+### Set up your GitHub Repository
+Fork [NeoFS REST Gateway upstream](https://github.com/nspcc-dev/neofs-rest-gw/fork) source
+repository to your own personal repository. Copy the URL of your fork (you will
+need it for the `git clone` command below).
+
+```sh
+$ git clone https://github.com/nspcc-dev/neofs-rest-gw
+```
+
+### Set up git remote as ``upstream``
+```sh
+$ cd neofs-rest-gw
+$ git remote add upstream https://github.com/nspcc-dev/neofs-rest-gw
+$ git fetch upstream
+$ git merge upstream/master
+...
+```
+
+### Create your feature branch
+Before making code changes, make sure you create a separate branch for these
+changes. Maybe you will find it convenient to name branch in
+`<type>/<Issue>-<changes_topic>` format.
+
+```
+$ git checkout -b feature/123-something_awesome
+```
+
+### Test your changes
+After your code changes, make sure
+
+- To add test cases for the new code.
+- To run `make lint`
+- To squash your commits into a single commit or a series of logically separated
+  commits run `git rebase -i`. It's okay to force update your pull request.
+- To run `make test` and `make all` completes.
+
+### Commit changes
+After verification, commit your changes. This is a [great
+post](https://chris.beams.io/posts/git-commit/) on how to write useful commit
+messages. Try following this template:
+
+```
+[#Issue] <component> Summary
+
+Description
+
+<Macros>
+
+<Sign-Off>
+```
+
+```
+$ git commit -am '[#123] Add some feature'
+```
+
+### Push to the branch
+Push your locally committed changes to the remote origin (your fork)
+```
+$ git push origin feature/123-something_awesome
+```
+
+### Create a Pull Request
+Pull requests can be created via GitHub. Refer to [this
+document](https://help.github.com/articles/creating-a-pull-request/) for
+detailed steps on how to create a pull request. After a Pull Request gets peer
+reviewed and approved, it will be merged.
+
+## DCO Sign off
+
+All authors to the project retain copyright to their work. However, to ensure
+that they are only submitting work that they have rights to, we are requiring
+everyone to acknowledge this by signing their work.
+
+Any copyright notices in this repository should specify the authors as "the
+contributors".
+
+To sign your work, just add a line like this at the end of your commit message:
+
+```
+Signed-off-by: Samii Sakisaka <samii@nspcc.ru>
+```
+
+This can easily be done with the `--signoff` option to `git commit`.
+
+By doing this you state that you can certify the following (from [The Developer
+Certificate of Origin](https://developercertificate.org/)):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```

README.md

@@ -82,3 +82,11 @@ details.
 You can see additional docs and swagger specification using the following url (suppose you ran rest-gw on `localhost:8090`):
 * http://localhost:8090/docs - rest-gw documentation
 * http://localhost:8090/v1/docs - swagger specification
+
+## Contributing
+
+Feel free to contribute to this project after reading the [contributing
+guidelines](CONTRIBUTING.md).
+
+Before starting to work on a certain topic, create a new issue first, describing
+the feature/topic you are going to implement.