Update modules/README.md

Author: jpnurmi

modules/README.md

@@ -15,3 +15,13 @@ To make sure we're not exposing any API from that library externally.
 Fork [getsentry/perfview](https://github.com/getsentry/perfview/).
 Tool from the .NET team which includes several utilities used for profiling .NET code. 
 We use that in our `Sentry.Profiling` package.
+
+### sentry-cocoa
+
+By default, `Sentry.Bindings.Cocoa` downloads a pre-built Sentry Cocoa SDK from
+GitHub Releases. The version is specified in `sentry-cocoa.properties`.
+
+If you want to build an unreleased Sentry Cocoa SDK version from source instead,
+replace the pre-built SDK with [getsentry/sentry-cocoa](https://github.com/getsentry/sentry-cocoa/)
+by cloning it into the `modules/sentry-cocoa` directory. To switch back to the
+pre-built SDK, delete the `modules/sentry-cocoa` directory.