Update media3 docs with details of new release branch

PiperOrigin-RevId: 430911179

README.md

@@ -94,7 +94,7 @@ to prevent build errors.
 
 Cloning the repository and depending on the modules locally is required when
 using some libraries. It's also a suitable approach if you want to make local
-changes, or if you want to use the main branch.
+changes, or if you want to use the `main` branch.
 
 First, clone the repository into a local directory:
 
@@ -127,7 +127,7 @@ implementation project(':media-lib-ui')
 Development work happens on the `main` branch. Pull requests should normally be
 made to this branch.
 
-We plan to add a release branch soon.
+The `release` branch holds the most recent stable release.
 
 #### Using Android Studio