Restore accidentally deleted link to building Robolectric

PiperOrigin-RevId: 437260678

README.md

@@ -58,7 +58,10 @@ Those software configurations are recommended and tested.
 - CMake 3.22.1+. Check it by `cmake --version`.
 - GCC 7.5.0+ on Linux or Apple clang 12.0.0+ on macOS. Check it by `gcc --version`.
 
-### build and test
+See [Building Robolectric](http://robolectric.org/building-robolectric/) for more details about setting up a build environment for Robolectric.
+
+### Build and test
+
 Robolectric supports running tests against multiple Android API levels. The work it must do to support each API level is slightly different, so its shadows are built separately for each. To build shadows for every API version, run:
 
     ./gradlew clean assemble testClasses --parallel