governance: add details on extension policy (envoyproxy#3460)

Fixes envoyproxy#2882

Signed-off-by: Matt Klein <mklein@lyft.com>

CONTRIBUTING.md

@@ -11,6 +11,8 @@ maximize the chances of your PR being merged.
   agreement. This is to prevent your time being wasted, as well as ours. The GitHub review process
   for major features is also important so that [organizations with commit access](OWNERS.md) can
   come to agreement on design.
+* Specifically, if the goal is to add a new [extension](REPO_LAYOUT.md#sourceextensions-layout),
+  please read the [extension policy](GOVERNANCE.md#extension-addition-policy).
 * Small patches and bug fixes don't need prior communication.
 
 # Coding style

GOVERNANCE.md

@@ -99,6 +99,12 @@ If a maintainer is no longer interested or cannot perform the maintainer duties
 should volunteer to be moved to emeritus status. In extreme cases this can also occur by a vote of
 the maintainers per the voting process below.
 
+# Extension addition policy
+
+Adding new [extensions](REPO_LAYOUT.md#sourceextensions-layout) has a dedicated policy. Please
+see [this](https://docs.google.com/document/d/1eDQQSxqx2khTXfa2vVm4vqkyRwXYkPzZCcbjxJ2_AvA) document
+for more information.
+
 # Conflict resolution and voting
 
 In general, we prefer that technical issues and maintainer membership are amicably worked out

README.md

@@ -2,7 +2,11 @@
 
 [C++ L7 proxy and communication bus](https://www.envoyproxy.io/)
 
-Envoy is hosted by the [Cloud Native Computing Foundation](https://cncf.io) (CNCF). If you are a company that wants to help shape the evolution of technologies that are container-packaged, dynamically-scheduled and microservices-oriented, consider joining the CNCF. For details about who's involved and how Envoy plays a role, read the CNCF [announcement](https://www.cncf.io/blog/2017/09/13/cncf-hosts-envoy/).
+Envoy is hosted by the [Cloud Native Computing Foundation](https://cncf.io) (CNCF). If you are a
+company that wants to help shape the evolution of technologies that are container-packaged,
+dynamically-scheduled and microservices-oriented, consider joining the CNCF. For details about who's
+involved and how Envoy plays a role, read the CNCF
+[announcement](https://www.cncf.io/blog/2017/09/13/cncf-hosts-envoy/).
 
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/1266/badge)](https://bestpractices.coreinfrastructure.org/projects/1266)