Add README.md.

Test: N/A
Change-Id: Iab6c979750682a1075d5a7c43f081458c91b7e7e

README.md

+# packages/modules/common
+
+This project includes mainline build and other utility code. Any library code
+intended for use by modules should go in `frameworks/libs/modules-utils`
+instead.
+
+## java code
+
+This project uses a single source path for java code. All java code should go
+in the `java` directory with subdirectories corresponding to the java package.
+`Android.bp` files should go alongside the java source files, and should only
+include java source for a single java package to encourage good code hygiene.
+
+Tests for java code should go in the `javatests` directory and follow the same
+structure.
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