Document @version syntax

[charles-cooper]

You can set a version pragma but it's undocumented. See test cases below for usage:
https://github.com/ethereum/vyper/blob/cdfb36d0c6d4a38ee3d9d34e001b141ec7c6f55f/tests/compiler/test_pre_parser.py#L74-L105

[charles-cooper]

here's an example:

# foo.vy
#@version0.1.13b

@public
def foo():
  pass

the version validator checks the major and minor version (but not the patch). so against compiler version v0.1.13b, @version0.1.13b or @version0.1.12 will validate but @version0.2.1 will not.

[fubuloubu]

There can be a space between version and the version number, right?