adds initial documentatin for custom runners

Signed-off-by: Robert Kielty <[email protected]>

ci/README.MD

@@ -1,7 +1,32 @@
-# External GitHub Action Runners   
+# External GitHub Action Runners
 
-** WORK IN PROGRESS **
+Maintained by the CNCF Projects Team, cncf/automation/ci orchestrates external
+GitHub Action Runners to run on resources donated to the CNCF.
 
-Maintained by the CNCF Projects Team, cncf/automation/ci orchestrates external GitHub Action Runners.
+CNCF Projects can make use of the External GitHub Action Runners that are
+defined here for CI jobs.
 
-CNCF Projects can make use of the External GitHub Action Runners that are defined here for CI jobs. 
+## Custom runners
+
+We have created two custom runners that allow us to run any containerizable CI
+job on both x86_64/amd64 and arm63 Architechtures.
+
+In a Github Action workflow, you can define multiple jobs and for each of those
+jobs you choose a runner. 
+
+We have defined two 16CPU/64GB runners that can used by adding either of the
+following statements to your job definitions:
+
+**For an amd64/x86_64 runner use:**
+
+`runs-on: oracle-16cpu-64gb-x86-64`
+
+**For an arm64 runner**
+
+`runs-on: oracle-16cpu-64gb-arm64`
+
+## Runner definition
+
+The above runners are defined in this repo <./gha-runner-image/Dockerfile>
+
+You can review the Dockerfile to see what tools have been added to the runner image.