Updates based on recent changes and discussion on RFC PR #262

[penpornk]

Changes to the Main RFC File

• Added the Implementation Conventions and Usage Overview section.
• Replaced the Stability / User Impact with the Versioning Strategy and Stability section. Also moved the section higher.
• Updated the API design according to the real implementation.
• Updated code examples.

Changes to the Versioning Strategy File:

Added/updated contents based on the discussion on the PluggableDevice RFC.

• Referenced semver and TensorFlow's Version Compatibility page.
• Added the Updating Guidelines, Convention, and Detecting Incompatibility sections.
• Merged the current code examples with @yisitu's newer ones on PR #262 and simplified them a bit.

Specifically, addressed the following points in the comments on the PluggableDevice RFC:

• Restructured the contents so the conventions/assumptions/procedures are clearer.
• Showed that the struct size check works with struct members in any position, not just the last member of the struct.
• Added more details on major version bump conventions. Specified that breaking changes require an RFC.
• Explicitly mentioned that nontrivial deprecations (that cannot use zero initialization) needs a major version bump.
• Added a requirement that fields that treat 0 and NULL as invalid values must have explicit comments saying so, to ensure all plug-ins have consistent behavior, e.g., none of them is using 0 or NULL for special cases.
• Updated the deprecation examples to use 'producer' and 'consumer' keywords (instead of plug-in and core TensorFlow) to cover more cases.

[penpornk]

@yisitu Could you please help review as well? (I can't tag you as a reviewer.) Thank you very much!

[yisitu]

I've done a round of review, please have a look. Thanks for consolidating the Github comments and polishing up the document Penporn!

[penpornk]

Thank you for the thorough review! :D

[yisitu]

LGTM