charms: update READMEs with sparse but accurate information

Author: Hyask

charms/README.md

@@ -0,0 +1,13 @@
+# Ubuntu Error Tracker charms
+
+Each folder each represent one charm, representing one component of the Error Tracker.
+
+## Testing locally
+
+In each folder, one can get the integration tests running by using the following commands:
+* `charmcraft pack`
+  This will pack the charm in its own folder. The tests have some magic fixtures
+  to find the right `.charm` files to use in the tests.
+* `uv run pytest tests/ -vv --log-level=INFO -o log_cli=1`
+  This is for maximum live verbosity. You don't actually need all those flags to
+  just run the tests, but I find that convenient in development.