Add instructions for using with subtrees.

Subtrees are the primary way I expect people to use the
`java-repo-tools` repository. Since it's not a super common workflow, I
document the needed steps in the README.

README.md

@@ -8,6 +8,169 @@ in the [GoogleCloudPlaftorm](https://github.com/GoogleCloudPlatform)
 organization.
 
 
+## Using this repository
+
+This repository is copied into a subtree of other Java repositories, such as
+[java-docs-samples](/GoogleCloudPlatform/java-docs-samples). Note, that a
+subtree is just the code copied into a directory, so a regular `git clone` will
+continue to work.
+
+
+### Adding to a new repository
+
+To copy `java-repo-tools` into a subtree of a new repository `my-java-samples`,
+first add this repository as a remote. We then fetch all the changes from this
+`java-repo-tools`.
+
+```
+git remote add java-repo-tools git@github.com:GoogleCloudPlatform/java-repo-tools.git
+git fetch java-repo-tools master
+```
+
+To make it easier to push changes back upstream, create a new branch.
+
+```
+git checkout -b java-repo-tools java-repo-tools/master
+```
+
+We can then go back to the `my-java-samples` code and prepare a Pull Request to
+add the `java-repo-tools` code in a subtree.
+
+```
+git checkout master
+# Making a new branch ia optional, but recommended to send a pull request to
+# start using java-repo-tools.
+git checkout -b use-java-repo-tools
+git read-tree --prefix=java-repo-tools/ -u java-repo-tools
+```
+
+Now all the content of `java-repo-tools` will be in the `java-repo-tools/`
+directory (which we specified in the `--prefix` command).
+
+#### Using the Maven configuration
+
+If all the projects within your `my-java-samples` share a common parent POM for
+plugin configuration (like checkstyle). We can then make the
+`java-repo-tools/pom.xml` parent of this.
+
+```
+<!-- Parent POM defines common plugins and properties. -->
+<parent>
+  <groupId>com.google.cloud</groupId>
+  <artifactId>shared-configuration</artifactId>
+  <version>1.0.0</version>
+  <relativePath>java-repo-tools</relativePath>
+</parent>
+```
+
+Once this is added to the common parent, all modules will have the same plugin
+configuration applied. If the children POMs provide the plugin information
+themselves, it will override this configuration, so you should delete any
+now-redundant plugin information.
+
+
+#### Examples
+
+- Adding to repository with an existing parent POM: Pull Request
+  [java-docs-samples#125][java-docs-samples-125].
+
+[java-docs-samples-125]: https://github.com/GoogleCloudPlatform/java-docs-samples/pull/125
+
+
+### Detecting if you need to synchronize a subtree
+
+If you haven't done this before, run
+
+```
+git remote add java-repo-tools
+git@github.com:GoogleCloudPlatform/java-repo-tools.git
+git fetch java-repo-tools master
+# Optional, but it makes pushing changes upstream easier.
+git checkout -b java-repo-tools java-repo-tools/master
+```
+
+To detect if you have changes in the directory, run
+
+```
+git fetch java-repo-tools
+git diff-tree -p HEAD:java-repo-tools/ java-repo-tools/master
+```
+
+or to diff against your local `java-repo-tools` branch:
+
+```
+git diff-tree -p HEAD:java-repo-tools/ java-repo-tools --
+```
+
+(The trailing `--` is to say that we want to compare against the branch, not the
+directory.)
+
+
+### Pulling changes from Java Repository Tools to a subtree
+
+To update the `java-repo-tools` directory, if you haven't done this before, run
+
+```
+git remote add java-repo-tools
+git@github.com:GoogleCloudPlatform/java-repo-tools.git
+git fetch java-repo-tools master
+git checkout -b java-repo-tools java-repo-tools/master
+```
+
+To pull the changes when in this branch run
+
+```
+git pull java-repo-tools master
+```
+
+To pull the changes back from upstream:
+
+```
+git checkout java-repo-tools
+git pull java-repo-tools master
+```
+
+Pull them into the main code.
+
+```
+git checkout master
+# Making a new branch is optional, but recommended to send a pull request for
+# update.
+git checkout -b update-java-repo-tools
+git merge --squash -Xsubtree=java-repo-tools/ --no-commit java-repo-tools
+```
+
+Then you can make any needed changes to make the rest of the repository
+compatible with the updated `java-repo-tools` code, commit, push, and send a
+Pull Request as you would in the normal flow.
+
+
+### Pushing changes from a subtree upstream to Java Repository Tools
+
+What if you make changes in your repository and now want to push them upstream?
+
+Assuming you just commited changes in the `java-repo-tools/` directory of your
+`my-main-branch`, to merge the changes into the local `java-repo-tools` branch,
+we need to cherry pick this commit using the subtree strategy. It will ignore
+any changes to file not in the `java-repo-tools/` directory.
+
+```
+git checkout java-repo-tools
+git cherry-pick -x --strategy=subtree my-main-branch
+```
+
+After you have committed all the changes you want to your `java-repo-tools`
+branch, you can push to the upstream `java-repo-tools` repository with the
+following command. (Replace `name-for-remote-branch` with the name you'd like to
+give the branch on the `java-repo-tools` repository.)
+
+```
+git push java-repo-tools java-repo-tools:name-for-remote-branch
+```
+
+Then, you can send a pull request to the `java-repo-tools` repository.
+
+
 ## Contributing changes
 
 -  See [CONTRIBUTING.md](CONTRIBUTING.md)