Doc changes to githubsource for specifying API URL (#949)

* Doc changes to githubsource for specifying API URL

This commit introduces extra documentation to support the new
functionality on githubsource where you can optionally specify
the baseURL of the github api server - thus enabling the existing
githubsource to be used against both public github and github
enterprise.  See https://github.com/knative/eventing-sources/pull/219

* Relative URL and quotes

eventing/README.md

@@ -177,6 +177,7 @@ The GitHubSource fires a new event for selected
 - `sink`:
   [ObjectReference](https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.12/#objectreference-v1-core)
   A reference to the object that should receive events.
+- `githubAPIURL`: `string` Optional field to specify the base URL for API requests. Defaults to the public GitHub API if not specified, but can be set to a domain endpoint to use with GitHub Enterprise, for example, `https://github.mycompany.com/api/v3/`. This base URL should always be specified with a trailing slash.
 
 See the [GitHub Source](samples/github-source) example.
 

eventing/samples/github-source/README.md

@@ -100,6 +100,10 @@ Source for a specific namespace. Be sure to replace the
 `ownerAndRepository` value with a valid GitHub public repository owned
 by your GitHub user.
 
+If using GitHub enterprise you will need to add an additional githubAPIURL
+field to the spec specifying your GitHub enterprise API endpoint, see 
+[here](../../README.md#githubsource)
+
 ```yaml
 apiVersion: sources.eventing.knative.dev/v1alpha1
 kind: GitHubSource

eventing/samples/github-source/github-source.yaml

@@ -18,3 +18,5 @@ spec:
     apiVersion: serving.knative.dev/v1alpha1
     kind: Service
     name: github-message-dumper
+# To use GitHub Enterprise you would need to add an entry for your githubAPIURL similar to the example below
+# githubAPIURL: "https://github.mycompany.com/api/v3/"