Add link to proto directory and update Contributing section (open-tel…

…emetry#130)

README.md

@@ -1,7 +1,7 @@
 # OpAMP Specification
 
 This repository contains Open Agent Management Protocol (OpAMP)
-[specification](specification.md).
+[specification](specification.md) and Protobuf definitions in [proto](proto) directory.
 
 See [releases here](https://github.com/open-telemetry/opamp-spec/releases) and [changelog here](CHANGELOG.md).
 
@@ -14,6 +14,10 @@ The specification is a [single markdown file](specification.md). If you make cha
 the section headings make sure to update the Table of Contents by running
 `make markdown-toc` in root directory.
 
+If any changes are made to Protobuf message definitions in the specification.md make
+sure to update also the `*.proto` files in the [proto](proto) directory. Run `make gen-proto`
+to run the Protobuf compiler and verify the `*.proto` files.
+
 Approvers ([@open-telemetry/opamp-spec-approvers](https://github.com/orgs/open-telemetry/teams/opamp-spec-approvers)):
 
 - [Alex Boten](https://github.com/codeboten), Lightstep