block.timestamp in different compiler versions

[vp-mazekine]

Added clarification on how block.timestamp works in different compiler version, based on the compiler changelog

[cryshado]

@vp-mazekine The API.md contains information about how the latest release of the compiler works without explaining the peculiarities of previous versions. If you are interested, just look at your assembly code to see how block.timestamp works on your compiler version. Thanks for the pull request, but we can't accept it