self-hosted-runner: document two more required secrets

To obtain the token that is necessary to register the runner, we need
some sort of credentials, and the way we do that is by using the
GitForWindowsHelper GitHub App's credentials. These have to be generated
from the App ID and the private key, therefore we require them as
secrets, which has previously not been documented.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/create-azure-self-hosted-runners.yml

@@ -50,6 +50,8 @@ env:
 # AZURE_RESOURCE_GROUP - Resource group to create the runner(s) in
 # AZURE_VM_USERNAME - Username of the VM so you can RDP into it
 # AZURE_VM_PASSWORD - Password of the VM so you can RDP into it
+# GH_APP_ID - The ID of the GitHub App whose credentials are to be used to obtain the runner token
+# GH_APP_PRIVATE_KEY - The private key of the GitHub App whose credentials are to be used to obtain the runner token
 jobs:
   create-runner:
     runs-on: ubuntu-latest