Proposal for 'How To' add SAI API Extensions (#315)

* Create SAI-extensions.docx

* Add files via upload

Describe how to add SAI API extensions

* Delete SAI-extensions.docx

* Added usage notes for SAI extensions

* Add explanations on using experimental features.

* Document version 1.3

   Clarify usage - with examples
   No SAI API changes that affect existing modules allowed for experimental attributes
   Mention test requirements