test/integration/README.md: Add notes on running tests (#3374)

Co-authored-by: UebelAndre <github@uebelandre.com>

test/integration/README.md

diff --git a/test/integration/README.md b/test/integration/README.md
index dbea623..138e49f 100644
--- a/test/integration/README.md
+++ b/test/integration/README.md
@@ -1,3 +1,10 @@
 # Integration
 
 Integration tests represented as their own workspaces.
+
+To run the tests, change to the respective subdirectory, then run `bazel` from there. For example:
+
+```sh
+cd test/integration/cc_common_link
+bazel test //...
+```