Guideline for release milestones and merging PRs (#1911)

Signed-off-by: Pavol Loffay <[email protected]>

CONTRIBUTING.md

@@ -142,3 +142,13 @@ a comment explaining why it is there, for example:
 $ cat ./pkg/cassandra/config/.nocover
 requires connection to Cassandra
 ```
+
+## Merging PRs
+Before merging a PR make sure:
+* the title is descriptive and follows [a good commit message](./CONTRIBUTING_GUIDELINES.md)
+* pull request is assigned to the current release milestone
+* add `changelog:*` and other labels
+
+Merge the PR by using "Squash and merge" option on Github. Avoid creating merge commits.
+After the merge make sure referenced issues were closed.
+

RELEASE.md

@@ -11,12 +11,13 @@
         git fetch
         git checkout {new_version} //e.g. v1.5.0
         ```
-2. After the PR is merged, create a release on Github:
+2. Add all merged pull requests to the milestone for the release and create a new milestone for a next release e.g. `Release 1.16`.
+3. After the PR is merged, create a release on Github:
     * Title "Release X.Y.Z" 
     * Tag `vX.Y.Z` (note the `v` prefix) and choose appropriate branch
     * Copy the new CHANGELOG.md section into the release notes
-3. The release tag will trigger a build of the docker images
-4. Once the images are available on [Docker Hub](https://hub.docker.com/r/jaegertracing/), announce the release on the mailing list, gitter, and twitter.
-5. Publish documentation for the new version in [jaegertracing.io](https://github.com/jaegertracing/documentation).
+4. The release tag will trigger a build of the docker images
+5. Once the images are available on [Docker Hub](https://hub.docker.com/r/jaegertracing/), announce the release on the mailing list, gitter, and twitter.
+6. Publish documentation for the new version in [jaegertracing.io](https://github.com/jaegertracing/documentation).
 
 Maintenance branches should follow naming convention: `release-major.minor` (e.g.`release-1.8`).