Add compatiblity tests against v2.5.0

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+# Protobuf Java Compatibility Tests
+
+This directory contains tests to ensure protobuf library is compatible with
+previously released versions.
+
+## Directory Layout
+
+For each released protobuf version we are testing compatiblity with, there
+is a sub-directory with the following layout (take v2.5.0 as an example):
+
+  * v2.5.0
+    * test.sh
+    * pom.xml
+    * protos/ - unittest protos.
+    * more_protos/ - unittest protos that import the ones in "protos".
+    * tests/ - actual Java test classes.
+
+The testing code is extracted from regular protobuf unittests by removing:
+
+  * tests that access package private methods/classes.
+  * tests that are known to be broken by an intended behavior change (e.g., we
+    changed the parsing recursion limit from 64 to 100).
+  * all lite runtime tests.
+
+It's also divided into 3 submodule with tests depending on more_protos and
+more_protos depending on protos. This way we can test scenarios where only part
+of the dependency is upgraded to the new version.
+
+## How to Run The Tests
+
+Before running the tests, make sure you have already built the protoc binary
+following [the C++ installation instructions](../../src/README.md). The test
+scripts will use the built binary located at ${protobuf}/src/protoc.
+
+To start a test, simply run the test.sh script in each version directory. For
+example:
+
+    $ v2.5.0/test.sh
+
+For each version, the test script will test:
+
+  * only upgrading protos to the new version
+  * only upgrading more_protos to the new version
+
+and see whether everything builds/runs fine. Both source compatibility and
+binary compatiblity will be tested.