Add instructions for running in mypy in top-level README

[twisted-trac]

	@rodrigc reported
Trac ID	trac#9872
Type	enhancement
Created	2020-06-22 23:37:27Z

It would be good to add info about running mypy to the top-level README.

Searchable metadata

trac-id__9872 9872
type__enhancement enhancement
reporter__rodrigc rodrigc
priority__normal normal
milestone__publish_types publish types
branch__ 
branch_author__ 
status__closed closed
resolution__fixed fixed
component__core core
keywords__review review
time__1592869047482102 1592869047482102
changetime__1594360403322407 1594360403322407
version__None None
owner__Craig_Rodrigues__rodrigc_____ Craig Rodrigues <rodrigc@...>

[twisted-trac]

	@rodrigc commented

#1317

[twisted-trac]

	@rodrigc set owner to @rodrigc @rodrigc set status to closed

In changeset bdfd80f

#!CommitTicketReference repository="" revision="bdfd80f63030b789fa693696e662a0dfea3779f9"
Merge pull request #1317 from twisted/9872-rodrigc-mypy

[mypy] Add lint and mypy instructions to README.rst

Author: rodrigc
Reviewer: adiroiban
Fixes: ticket:9872