documentation for version support

[hyehudai]

please explicitly document version support.

• should i, as extension owner, do something to support any future releases ?
  e.g. i am releasing at v1.4.3 - will it work for 1.4.2 / 1.4.4 ? / 1.5.0?

• can older releases still install my extension ( assuming it supported it )
  e.g. i support release 1.4.3 - will someone running v1.4.3 , when 1.4.5 is out can still install my extension ? and when 1.5.0 is out ?