Add a requirements.txt to repo.

[diptorupd]

Let us provide the specific versions of each of these dependencies if not a range of versions

Originally posted by @reazulhoque in #397 (comment)

[diptorupd]

Also update quick start guide once requirements.txt is added.

[oleksandr-pavlyk]

The requirements.txt was added in #540

Specific versions were not pinned though.