Add an HOWTO for releasing Gitea

In the hope it'll be useful for as a policy for the future

HOWTO_RELEASE.md

@@ -0,0 +1,14 @@
+Release procedure is as follows:
+
+- Let $vmaj, $vmin and $vpat be Major, Minor and Patch version numbers
+- Drop "dev" suffix from $vpat
+- Let $ver be "$vmaj.$vmin.$vpat"
+- Make sure Version`variable is set correctly in main.go ($ver)
+- Compile CHANGELOG.md section for $ver
+  (please only include user-relevant changes, and be concise)
+- git tag -a v$ver
+- If this is a .0 release:
+  - git branch release/v$vmaj.$vmin
+  - Update Version in new branch ( vpat++ )
+- Update Version in master branch ( vmin++; vpat=0-dev )
+- Send PR to https://github.com/go-gitea/blog announcing the release