Add a "Contributing" section to README.md .

# Soong

Soong is the replacement for the old Android make-based build system.  It

replaces Android.mk files with Android.bp files, which are JSON-like simple

declarative descriptions of modules to build.

Soong is one of the build systems used in Android. There are altogether three:

* The legacy Make-based build system that is controlled by files called

  `Android.mk`.

* Soong, which is controlled by files called `Android.bp`.

* The upcoming Bazel-based build system that is controlled by files called

  `BUILD.bazel`.

`Android.bp` file are JSON-like declarative descriptions of "modules" to build;

a "module" is the basic unit of building that Soong understands, similarly to

how "target" is the basic unit of building for Bazel (and Make, although the

two kinds of "targets" are very different)

See [Simple Build

Configuration](https://source.android.com/compatibility/tests/development/blueprints)

on source.android.com to read how Soong is configured for testing.

### Contributing

Code reviews are handled through the usual code review system of Android,

available [here](https://android-review.googlesource.com/dashboard/self).

For simple changes (fixing typos, obvious optimizations, etc.), sending a code

review request is enough. For more substantial changes, file a bug in our

[bug tracker](https://issuetracker.google.com/issues/new?component=381517) or

or write us at android-building@googlegroups.com .

For Googlers, see our [internal documentation](http://go/soong).

## Android.bp file format

By design, Android.bp files are very simple.  There are no conditionals or