document dict-comp and genexpr
document set-comp
Merge master onto pr/580
Typo fixes in comments

Signed-off-by: Kartik Mistry <kartik.mistry@gmail.com>
Merge branch 'master' into pr/586
Move contributing from hacking docs to repo root

A CONTRIBUTING.rst is added to the root of the repo, as Github
automatically recognizes this for incoming PRs
(ref: https://github.com/blog/1184-contributing-guidelines)

Most of the information is directly borrowed from the existing hacking
docs, which is restructured to refer to this file instead for
contribution guidelines
Merge pull request #587 from theanalyst/docs/contributing

Move contributing from hacking docs to repo root
Add dockerfile.
the mega typo fix commit

Wow! much typos! such fix!
Merge pull request #592 from theanalyst/fix/typo

the mega typo fix commit
Merge pull request #590 from paultag/paultag/dockerfile

Add dockerfile.
Display python version & platform at repl startup

*hy/cmdline.py: At the repl startup the Python implementation & version
 is displayed along with the platform(os) information