Add compatibility notice for Java.

java/README.md

@@ -17,9 +17,10 @@ rather build without Maven, see below.
 
      http://maven.apache.org/
 
-2) Build the C++ code, or obtain a binary distribution of protoc.  If
-   you install a binary distribution, make sure that it is the same
-   version as this package.  If in doubt, run:
+2) Build the C++ code, or obtain a binary distribution of protoc (see
+   the toplevel [README.md](../README.md)). If you install a binary
+   distribution, make sure that it is the same version as this package.
+   If in doubt, run:
 
      $ protoc --version
 
@@ -44,36 +45,25 @@ rather build without Maven, see below.
 
    The .jar will be placed in the "target" directory.
 
-Installation - 'Lite' Version - With Maven
-==========================================
-
-Building the 'lite' version of the Java Protocol Buffers library is
-the same as building the full version, except that all commands are
-run using the 'lite' profile.  (see
-http://maven.apache.org/guides/introduction/introduction-to-profiles.html)
-
-E.g. to install the lite version of the jar, you would run:
+The above instructions will install 3 maven artifacts:
 
-    $ mvn install -P lite
-
-The resulting artifact has the 'lite' classifier.  To reference it
-for dependency resolution, you would specify it as:
-
-```
-  <dependency>
-    <groupId>com.google.protobuf</groupId>
-    <artifactId>protobuf-java</artifactId>
-    <version>${version}</version>
-    <classifier>lite</classifier>
-  </dependency>
-```
+  * protobuf-java: The core Java Protocol Buffers library. Most users only
+                   need this artifact.
+  * protobuf-lite: The lite version of core Java Protobuf Buffers library. It
+                   is a subset of the core library and is used together with
+                   the 'lite' code generator flag to reduce generated code size
+                   for mobile.
+  * protobuf-java-util: Utilities to work with protos. It contains JSON support
+                        as well as utilities to work with proto3 well-known
+                        types.
 
 Installation - Without Maven
 ============================
 
 If you would rather not install Maven to build the library, you may
 follow these instructions instead.  Note that these instructions skip
-running unit tests.
+running unit tests and only describes how to install the core protobuf
+library (without the util package).
 
 1) Build the C++ code, or obtain a binary distribution of protoc.  If
    you install a binary distribution, make sure that it is the same
@@ -86,15 +76,50 @@ running unit tests.
 
 2) Invoke protoc to build DescriptorProtos.java:
 
-     $ protoc --java_out=src/main/java -I../src \
+     $ protoc --java_out=core/src/main/java -I../src \
          ../src/google/protobuf/descriptor.proto
 
-3) Compile the code in src/main/java using whatever means you prefer.
+3) Compile the code in core/src/main/java using whatever means you prefer.
 
 4) Install the classes wherever you prefer.
 
-Usage
-=====
+Compatibility Notice
+====================
+
+* Protobuf minor version releases are backwards-compatible. If your code
+  can build/run against the old version, it's expected to build/run against
+  the new version as well. Both binary compatibility and source compatbility
+  are guaranteed for minor version releases if the user follows the guideline
+  described in this section.
+
+* Protobuf major version releases may also be backwards-compatbile with the
+  last release of the previous major version. See the release notice for more
+  details.
+
+* APIs marked with the @ExperimentalApi annotation are subject to change. They
+  can be modified in any way, or even removed, at any time. Don't use them if
+  compatiblity is needed. If your code is a library itself (i.e. it is used on
+  the CLASSPATH of users outside your own control), you should not use
+  experimental APIs, unless you repackage them (e.g. using ProGuard).
+
+* Deprecated non-experimental APIs will be removed two years after the release
+  in which they are first deprecated. You must fix your references before this
+  time. If you don't, any manner of breakage could result (you are not
+  guaranteed a compilation error).
+
+* Protobuf message interfaces/classes are designed to be subclassed by protobuf
+  generated code only. Do not subclass these message interfaces/classes
+  yourself. We may add new methods to the message interfaces/classes which will
+  break your own subclasses.
+
+* Don't use any method/class that is marked as "used by generated code only".
+  Such methods/classes are subject to change.
+
+* Protobuf LITE runtime APIs are not stable yet. They are subject to change even
+  in minor version releases.
+
+Documentation
+=============
 
 The complete documentation for Protocol Buffers is available via the
 web at: