CHANGELOG: add 0.15.0 release

CHANGELOG.md

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+# Changelog
+
+This document outlines major changes between releases.
+
+## 0.15.0 (30 Apr 2021)
+
+This is the first public release incorporating latest NeoFS protocol support
+and fixing some bugs.
+
+New features:
+ * upload support (#14, #13, #29)
+ * ephemeral keys (#26)
+ * TLS server support (#28)
+
+Behavior changes:
+ * node weights can now be specified as simple numbers instead of percentages
+   and gateway will calculate the proportion automatically (#27)
+ * attributes are converted now to `X-Attribute-*` headers when retrieving
+   object from gate instead of `X-*` (#29)
+
+Improvements:
+ * better Makefile (#16, #24, #33)
+ * updated documentation (#16, #29)
+ * updated neofs-api-go to v1.25.0 (#17, #20)
+ * updated fasthttp to v1.23.0+ (#17, #29)
+ * refactoring, eliminating some dependencies (#20, #29)
+
+Bugs fixed:
+ * gateway attempted to work with no NeoFS peers configured (#29)
+ * some invalid headers could be sent for attributes using non-ASCII or
+   non-printable characters (#29)
+
+## Older versions
+
+Please refer to [Github
+releases](https://github.com/nspcc-dev/neofs-http-gate/releases/) for older
+releases.