Document how to manually run the CI scripts

PiperOrigin-RevId: 417719589

Author: fredrec

ci/kokoro/README.md

@@ -1 +1,15 @@
 The entrypoint build script is gcp_ubuntu/build.sh.
+
+
+To manually trigger the unit test run as the CI system does:
+
+```
+export GIT_REPO_DIR=<absolute path to the git repo>
+export KOKORO_ARTIFACTS_DIR="${GIT_REPO_DIR}"
+ci/kokoro/gcp_ubuntu/build.sh
+```
+
+That is, if the current directory is the root of the Git repository:
+```
+GIT_REPO_DIR="`pwd`" KOKORO_ARTIFACTS_DIR="`pwd`" ci/kokoro/gcp_ubuntu/build.sh
+```