Update pkg/README.md with how to add new packages, run tests

Change-Id: Idfab3de5ebc07e2acf084de43da02a0bee6851e3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415863
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

pkg/README.md

@@ -42,3 +42,14 @@ We validate:
 - that the dependencies listed in the pubspec are used in the package
 - that all the packages used by the source are listed in the pubspec
 - that a reference to a pkg/ package is done via a relative path dependency
+
+# Adding new packages
+
+Run `gclient sync` to re-generate `.dart_tool/package_config.json` after adding
+a new package.
+
+# Testing packages
+
+Currently `dart test` can't be used to run the tests, use `test.py` as usual.
+E.g. the command `./tools/test.py -n...<path>` runs all tests with prefix
+`<path>`.