contrib: adds a guide to make a pull request. (#746)

CONTRIBUTING.md

@@ -31,6 +31,50 @@ a lot of time on something the project's developers might not want to merge into
 **IMPORTANT**: By submitting a patch, you agree to allow the project
 owners to license your work under the terms of the [MIT License](LICENSE).
 
+### How to create a pull request
+
+First, you have to install [GoLang](https://golang.org/doc/install) and [golangci-lint](https://github.com/golangci/golangci-lint#install).
+
+```bash
+# Create the root folder
+mkdir -p $GOPATH/src/github.com/xenolf
+cd $GOPATH/src/github.com/xenolf
+
+# clone your fork
+git clone git@github.com:YOUR_USERNAME/lego.git
+cd lego
+
+# Add the xenolf/lego remote
+git remote add upstream git@github.com:xenolf/lego.git
+git fetch upstream
+```
+
+```bash
+# Create your branch
+git checkout -b my-feature
+
+## Create your code ##
+```
+
+```bash
+# Format
+make fmt
+# Linters
+make checks
+# Tests
+make test
+# Compile
+make build
+```
+
+```bash
+# push your branch
+git push -u origin my-feature
+
+## create a pull request on Github ##
+```
+
+
 ## DNS Providers: API references
 
 | DNS provider              | Code           | Documentation                                                                                                | Go client                                                         |