Add a quick tour of the project to CONTRIBUTING (#30187)

Adds a description of the most important subdirectories to
`CONTRIBUTING.md` to help folks that are less familiar with the project
get their bearings. It reflects that state of the project right now so
it will inevitably go out of date. But I'll try and keep it up to date.

CONTRIBUTING.md

@@ -209,6 +209,95 @@ Before submitting your changes, run the test suite to make sure that nothing is
 ./gradlew check
 ```
 
+### Project layout
+
+This repository is split into many top level directories. The most important
+ones are:
+
+#### `docs`
+Documentation for the project.
+
+#### `distribution`
+Builds our tar and zip archives and our rpm and deb packages.
+
+#### `libs`
+Libraries used to build other parts of the project. These are meant to be
+internal rather than general purpose. We have no plans to
+[semver](https://semver.org/) their APIs or accept feature requests for them.
+We publish them to maven central because they are dependencies of our plugin
+test framework, high level rest client, and jdbc driver but they really aren't
+general purpose enough to *belong* in maven central. We're still working out
+what to do here.
+
+#### `modules`
+Features that are shipped with Elasticsearch by default but are not built in to
+the server. We typically separate features from the server because they require
+permissions that we don't believe *all* of Elasticsearch should have or because
+they depend on libraries that we don't believe *all* of Elasticsearch should
+depend on.
+
+For example, reindex requires the `connect` permission so it can perform
+reindex-from-remote but we don't believe that the *all* of Elasticsearch should
+have the "connect". For another example, Painless is implemented using antlr4
+and asm and we don't believe that *all* of Elasticsearch should have access to
+them.
+
+#### `plugins`
+Officially supported plugins to Elasticsearch. We decide that a feature should
+be a plugin rather than shipped as a module because we feel that it is only
+important to a subset of users, especially if it requires extra dependencies.
+
+The canonical example of this is the ICU analysis plugin. It is important for
+folks who want the fairly language neutral ICU analyzer but the library to
+implement the analyzer is 11MB so we don't ship it with Elasticsearch by
+default.
+
+Another example is the `discovery-gce` plugin. It is *vital* to folks running
+in [GCP](https://cloud.google.com/) but useless otherwise and it depends on a
+dozen extra jars.
+
+#### `qa`
+Honestly this is kind of in flux and we're not 100% sure where we'll end up.
+Right now the directory contains
+* Tests that require multiple modules or plugins to work
+* Tests that form a cluster made up of multiple versions of Elasticsearch like
+full cluster restart, rolling restarts, and mixed version tests
+* Tests that test the Elasticsearch clients in "interesting" places like the
+`wildfly` project.
+* Tests that test Elasticsearch in funny configurations like with ingest
+disabled
+* Tests that need to do strange things like install plugins that thrown
+uncaught `Throwable`s or add a shutdown hook
+But we're not convinced that all of these things *belong* in the qa directory.
+We're fairly sure that tests that require multiple modules or plugins to work
+should just pick a "home" plugin. We're fairly sure that the multi-version
+tests *do* belong in qa. Beyond that, we're not sure. If you want to add a new
+qa project, open a PR and be ready to discuss options.
+
+#### `server`
+The server component of Elasticsearch that contains all of the modules and
+plugins. Right now things like the high level rest client depend on the server
+but we'd like to fix that in the future.
+
+#### `test`
+Our test framework and test fixtures. We use the test framework for testing the
+server, the plugins, and modules, and pretty much everything else. We publish
+the test framework so folks who develop Elasticsearch plugins can use it to
+test the plugins. The test fixtures are external processes that we start before
+running specific tests that rely on them.
+
+For example, we have an hdfs test that uses mini-hdfs to test our
+repository-hdfs plugin.
+
+#### `x-pack`
+Commercially licensed code that integrates with the rest of Elasticsearch. The
+`docs` subdirectory functions just like the top level `docs` subdirectory and
+the `qa` subdirectory functions just like the top level `qa` subdirectory. The
+`plugin` subdirectory contains the x-pack module which runs inside the
+Elasticsearch process. The `transport-client` subdirectory contains extensions
+to Elasticsearch's standard transport client to work properly with x-pack.
+
+
 Contributing as part of a class
 -------------------------------
 In general Elasticsearch is happy to accept contributions that were created as