b/143632250: Cleanup README
b/143632250: Cleanup README

- Official Name: "API Proxy"
- Add a little more info and links
- Add release info
- Add disclaimer

Change-Id: Iefd006d75929d26e920ae395db8d6aaafd5da602
Signed-off-by: Teju Nareddy <86db88dbc1cba831353c118b8473a42f6361b545@google.com>

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