Add release procedure to docs

docs/RELEASE.md

@@ -0,0 +1,23 @@
+# Release procedure
+
+Only `master` branch is used for releases.
+
+1. Write a **draft** of release where _tag version_ and _release title_ are
+  the same as a version of `hadolint`, eg `v1.2.3`.
+  ![draft](https://user-images.githubusercontent.com/18702153/32983073-f7477820-cc86-11e7-92c6-fabfc1223a25.png)
+
+2. Create an annotated tag with tag name as a message and push it back to
+ `lukasmartinelli/hadolint` remote. Release notes should be in the draft not
+ in a tag message.
+  ```bash
+  export TAG=v1.2.3
+  git tag -a "$TAG" -m "$TAG" && git push origin "$TAG"
+  ```
+
+3. Tag creation will trigger build in _Travis_ and _AppVeyor_ which will upload
+  binaries to GitHub release draft with the same name as is the name of the tag
+  (created in step 1).
+  ![draft_binaries](https://user-images.githubusercontent.com/18702153/32983247-7692d528-cc89-11e7-9340-2af1434a6bdf.png)
+
+4. Edit release notes, mention all new features and important fixes and
+  publish it.