README: add compatibility section
diff --git a/README.md b/README.md
index 8fdc89b..037fc7c 100644
--- a/README.md
+++ b/README.md
@@ -192,7 +192,49 @@
protoc --go_out=plugins=grpc:. *.proto
-## Plugins ##
+## Compatibility ##
+
+The library and the generated code are expected to be stable over time.
+However, we reserve the right to make breaking changes without notice for the
+following reasons:
+
+- Security. A security issue in the specification or implementation may come to
+ light whose resolution requires breaking compatibility. We reserve the right
+ to address such security issues.
+- Unspecified behavior. There are some aspects of the Protocol Buffers
+ specification that are undefined. Programs that depend on such unspecified
+ behavior may break in future releases.
+- Specification errors or changes. If it becomes necessary to address an
+ inconsistency, incompleteness, or change in the Protocol Buffers
+ specification, resolving the issue could affect the meaning or legality of
+ existing programs. We reserve the right to address such issues, including
+ updating the implementations.
+- Bugs. If the library has a bug that violates the specification, a program
+ that depends on the buggy behavior may break if the bug is fixed. We reserve
+ the right to fix such bugs.
+- Adding methods or fields to generated structs. These may conflict with field
+ names that already exist in a schema, causing applications to break. When the
+ code generator encounters a field in the schema that would collide with a
+ generated field or method name, the code generator will append an underscore
+ to the generated field or method name.
+- Adding, removing, or changing methods or fields in generated structs that
+ start with `XXX`. These parts of the generated code are exported out of
+ necessity, but should not be considered part of the public API.
+- Adding, removing, or changing unexported symbols in generated code.
+
+Any breaking changes outside of these will be announced 6 months in advance to
+protobuf@googlegroups.com.
+
+You should, whenever possible, use generated code created by the `protoc-gen-go`
+tool built at the same commit as the `proto` package. The `proto` package
+declares package-level constants in the form `ProtoPackageIsVersionX`.
+Application code and generated code may depend on one of these constants to
+ensure that compilation will fail if the available version of the proto library
+is too old. Whenever we make a change to the generated code that requires newer
+library support, in the same commit we will increment the version number of the
+generated code and declare a new package-level constant whose name incorporates
+the latest version number. Removing a compatibility constant is considered a
+breaking change and would be subject to the announcement policy stated above.
The `protoc-gen-go/generator` package exposes a plugin interface,
which is used by the gRPC code generation. This interface is not
