Add updates to agent charm README for debugging methods

agent/charms/testflinger-agent-charm/README.md

@@ -1,10 +1,13 @@
 # Overview
 
-This is the charm to deploy the testflinger-agent project.  You can find
-the source for testflinger at: https://github.com/canonical/testflinger-agent
+This is the charm to deploy Testflinger agents to an agent host.  You can find
+the source for testflinger at: https://github.com/canonical/testflinger
 
 The source for testflinger-agent will be pulled directly from git trunk on the
-project listed above, for now.
+project listed above. This creates a sparse/shallow checkout of testflinger
+which only includes the "agent" and "device-connectors" directories since it
+doesn't require all of the other parts of the monorepo, and this keeps
+the space used for the git checkout to a minimum.
 
 # Building
 To build this charm, first install charmcraft (sudo snap install --classic
@@ -21,3 +24,15 @@ Supported options for this charm are:
       base64 encoded string with the config file for spi-agent
   - device-configfile:
       base64 encoded string with the config file for snappy-device-agents
+
+# Debugging
+For local debugging, it might be useful to switch branches and pull updates
+in order to test or debug new code. However, because we only clone the
+minimum required things from git, you will not see other branches
+automatically. In order to change your local clone so that you will see
+everythiing, run:
+
+   ```bash 
+   $ git remote set-branches origin '*'
+   $ git fetch origin
+   ```