Update ReadMe to reflect quality levels of client libraries

[omaray]

We will soon have client libraries at different quality levels. Let's make sure we reflect this information in our top-level ReadMe file the moment we cut the new release.

The update for PHP's ReadMe has already be done (as a Pull Request for now) and can be used as the canonical example to follow. The list below highlights the main changes to follow for this repo.

General

• Remove the text "This project supports the following Google cloud Platform services:"

Listing Beta client libraries

• Create a section to show the Beta client libraries (see image right below for text & layout)

• Make the term Beta link to the Versioning section of the ReadMe (see image right below)

• Add the term (Beta) next to each API name (see image right below for text and visual):

Listing Alpha client libraries

• Create a section to show the Alpha client libraries (see image right below for text and layout)

• Make the term Alpha link to the Versioning section of the ReadMe (see image right below)

• Add the term (Alpha) next to each API name (see image right below for text and visual):

Client Library per API section

• If there is a section in the ReadMe for each API, add the term (Beta) or (Alpha) appropriately (see image right below for visual):

Versioning section

• If the Versioning section doesn't exist, add one.

• Add the following content in the Versioning section:

[stephenplusplus]

PR sent: #1864