README.md: add a section on contributing patches

README.md

@@ -35,3 +35,9 @@ Link against the `protobuf-c` library.
 This will place compiler flags in the `PROTOBUF_C_CFLAGS` variable and linker flags in the `PROTOBUF_C_LDFLAGS` variable.
 
 (Note that the `protobuf-c` header file used to be installed as `google/protobuf-c/protobuf-c.h` in previous versions.)
+
+## Contributing
+
+Please send patches to the [protobuf-c mailing list](https://groups.google.com/forum/#!forum/protobuf-c) or by opening a GitHub pull request.
+
+Copyright to all contributions are retained by the original author, but must be licensed under the terms of the [BSD-2-Clause](http://opensource.org/licenses/BSD-2-Clause) license. Please add a `Signed-off-by` header to your commit message (`git commit -s`) to indicate that you are licensing your contribution under these terms.