Create README to explain the repo

To help communicate the purpose of the repo, how to use it, and details
around its structure, I've created a README explain those key points.

I am only adding it to the main branch since the test branch does not
need that documentation since the test branch, by design, is free of
restrictions.

README.md

@@ -0,0 +1,33 @@
+# Widevine SCP License
+
+This repo contains the current SCP license used across all Widevine projects.
+This file must be installed on any machine attempting to build a project with
+SCP.
+
+
+## Using the repo
+
+This repo should always contain a valid and up-to-date license. A
+non-machine-specific license expires every 30 days. To prevent the license from
+expiring there is a Kokoro job that will replace the license with a new license
+two a month.
+
+The license must be in the partner repo to allow the Kokoro bot write access to
+it since Kokoro jobs (once running) do not have corp access.
+
+## Using the license
+
+Your project should contain a depend on the repo at `HEAD` so that you always
+have the latest license. To install the license on your machine, call:
+
+```
+scp-tool --set-license <<license file>>
+```
+
+## Branches
+
+`main` is where the valid license will be stored. You should only depend on this
+branch unless there is a specific reason not to.
+
+`test` is primarily used when testing bot logic. It allows for changes to bot
+scripts without breaking anyone using the license on `main`.