Add instruction about release branches (#185)

* Add instruction about releas branches

* Change SNAPSHOT to dev

* Update release notes with instructions to build and publish release notes.

Author: Bogdan Drutu

RELEASING.md

@@ -0,0 +1,93 @@
+# How to Create a Release of OpenCensus C++ (for Maintainers Only)
+
+## Tagging the Release
+
+The first step in the release process is to create a release branch, bump
+versions, and create a tag for the release. Our release branches follow the
+naming convention of `v<major>.<minor>.x`, while the tags include the patch
+version `v<major>.<minor>.<patch>`. For example, the same branch `v0.1.x` would
+be used to create all `v0.1` tags (e.g. `v0.1.0`, `v0.1.1`).
+
+In this section upstream repository refers to the main opencensus-cpp github
+repository.
+
+Before any push to the upstream repository you need to create a [personal access
+token](https://help.github.com/articles/creating-a-personal-access-token-for-the-command-line/).
+
+1.  Create the release branch and push it to GitHub:
+
+    ```bash
+    $ MAJOR=0 MINOR=1 PATCH=0 # Set appropriately for new release
+    $ VERSION_FILES=(
+        opencensus/common/version.h
+      )
+    $ git checkout -b v$MAJOR.$MINOR.x master
+    $ git push upstream v$MAJOR.$MINOR.x
+    ```
+
+    The branch will be automatically protected by the GitHub branch protection rule for release
+    branches.
+
+2.  For `master` branch:
+
+    -   Change version files to the next minor snapshot (e.g. `0.2.0-dev`).
+
+    ```bash
+    $ git checkout -b bump-version master
+    # Change version to next minor (and keep -dev)
+    $ sed -i 's/\(.*OPENCENSUS_VERSION.*\)[0-9]\+\.[0-9]\+\.[0-9]\+/\1'$MAJOR.$((MINOR+1)).0'/' \
+      "${VERSION_FILES[@]}"
+    $ tools/presubmit.sh
+    $ git commit -a -m "Start $MAJOR.$((MINOR+1)).0 development cycle"
+    ```
+
+    -   Go through PR review and push the master branch to GitHub:
+
+    ```bash
+    $ git checkout master
+    $ git merge --ff-only bump-version
+    $ git push upstream master
+    ```
+
+3.  For `vMajor.Minor.x` branch:
+
+    -   Change version files to remove "-dev" for the next release
+        version (e.g. `0.4.0`). Commit the result and make a tag:
+
+    ```bash
+    $ git checkout -b release v$MAJOR.$MINOR.x
+    # Change version to remove -dev
+    $ sed -i 's/\(.*OPENCENSUS_VERSION.*[0-9]\+\.[0-9]\+\.[0-9]\+\)-dev/\1/' \
+      "${VERSION_FILES[@]}"
+    $ tools/presubmit.sh
+    $ git commit -a -m "Bump version to $MAJOR.$MINOR.$PATCH"
+    $ git tag -a v$MAJOR.$MINOR.$PATCH -m "Version $MAJOR.$MINOR.$PATCH"
+    ```
+
+    -   Change root build files to the next snapshot version (e.g.
+        `0.4.1-dev`). Commit the result:
+
+    ```bash
+    # Change version to next patch and add -dev
+    $ sed -i 's/\(.*OPENCENSUS_VERSION.*\)[0-9]\+\.[0-9]\+\.[0-9]\+/\1'$MAJOR.$((MINOR+1)).$((PATCH+1))-dev'/' \
+      "${VERSION_FILES[@]}"
+    $ tools/presubmit.sh
+    $ git commit -a -m "Bump version to $MAJOR.$MINOR.$((PATCH+1))-dev"
+    ```
+
+    -   Go through PR review and push the release tag and updated release branch
+        to GitHub:
+
+    ```bash
+    $ git checkout v$MAJOR.$MINOR.x
+    $ git merge --ff-only release
+    $ git push upstream v$MAJOR.$MINOR.$PATCH
+    $ git push upstream v$MAJOR.$MINOR.x
+    ```
+
+4. Write release notes
+   -   Go to the [release page][RELEASE_LINK], draft a new release and publish
+       it after review.
+
+
+[RELEASE_LINK]: https://github.com/census-instrumentation/opencensus-cpp/releases