b/138682814: revise root README
b/138682814: revise root README

- replace absolute path to releative path
- remove google internal projects
- nest the image in the md
- nit

Change-Id: Ia5b9e7c7753bd2ae4b58ae6509277beeacb4545c

GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2