Update documentation for master branch

With this change, master branch has been moved to point
at the most recent custom looker branch.  This was done
to avoid scanning vulnerabilities for puma that are on
master but not on the version that Looker uses.

b/302719604

Change-Id: Ida6b63a21a05e144d84b10c155cdd8cb5cb9c313
(cherry picked from commit 4156506)

deploying.md

@@ -148,26 +148,36 @@ Choose "Browse" then the "looker-gems-private" repository. Press the
 "Upload component" button, then browse for the gem file you wish to
 upload and choose the "Upload" button.
 
-### Update Helltool
-
-Modify the `Gemfile` to point to the correct version with the correct
-source like the following example.
-```
-  gem 'gemname', 'X.Y.Z.looker.custom',
-    :source => 'https://nexusrepo.looker.com/repository/looker-gems-private/'
-```
+### Master Branch
 
-Now run `bin/prepare` and verify that everything works locally. Commit the
-changed `Gemfile` and `Gemfile.lock` and push the changes to Gerrit like
-normal.
+The master branch has now been moved to point at the most recent version of this
+library that Helltool uses.  Whenever this branch is updated, master branch
+should also move accordingly.  Master should always be current.
 
 ### Update the Gem Project
 
 Commit the changes and push to Gerrit with
+
 ```
 git push origin HEAD:refs/for/master
 # or
 git push origin HEAD:refs/for/branch_name
 ```
 
-Have the changes reviewed and accepted.
+Have the changes reviewed and accepted.  Most new updates should target the
+master branch, but when you are moving to a new version, please add a new
+branch to track the old version.
+
+### Update Helltool
+
+Modify the `Gemfile` in Helltool to point to the correct version with the 
+correct source like the following example.
+
+```
+  gem 'gemname', 'X.Y.Z.looker.custom',
+    :source => 'https://nexusrepo.looker.com/repository/looker-gems-private/'
+```
+
+Now run `bin/prepare` and verify that everything works locally. Commit the
+changed `Gemfile` and `Gemfile.lock` and push the changes to Gerrit like
+normal.