Expanded contribution guidelines
Expanded contribution guidelines

- Added IETF syntax: MUST, SHOULD, etc.
- Added items for:
  - file names
  - PHP references
  - HTTP messages
  - Image "alt" tag syntax
  - Tables
  - Lists

santiph/apigility-documentation,poisa/apigility-documentation,PowerKiKi/apigility-documentation,ezimuel/apigility-documentation