Document process for bumping dev-master branch-alias

Author: jmikola

CONTRIBUTING.md

@@ -124,9 +124,38 @@ The maintenance branch's HEAD will be the target for our release tag:
 $ git tag -a -m "Release X.Y.Z" X.Y.Z
 ```
 
-### Push tags
+### Update branch-alias for dev-master (major and minor releases only)
+
+Initial versions of new major or minor releases will likely be made on the
+master branch directly instead of a maintenance branch. In this case, the
+`branch-alias` for `dev-master` in `composer.json` should be updated. For
+example, after tagging a new 1.10.0 release on the master branch,
+`composer.json` in the master branch may still read:
+
+```
+"branch-alias": {
+    "dev-master": "1.9.x-dev"
+}
+```
+
+The above would be changed to:
+
+```
+"branch-alias": {
+    "dev-master": "1.10.x-dev"
+}
+```
+
+Commit this change:
 
 ```
+$ git commit -m "Master is now 1.10-dev" composer.json
+```
+
+### Push commits and tags
+
+```
+$ git push
 $ git push --tags
 ```