Rename CONTRIBUTING to CONTRIBUTING.md

[twisted-trac]

	@twm reported
Trac ID	trac#9916
Type	enhancement
Created	2020-07-18 21:34:10Z
Branch	https://github.com/twisted/twisted/tree/contributing-9616

This would make links clickable when viewing the file on GitHub.

Also, update the language around Travis CI, buildbot, and codecov.io.

Searchable metadata

trac-id__9916 9916
type__enhancement enhancement
reporter__twm twm
priority__normal normal
milestone__None None
branch__contributing_9616 contributing-9616
branch_author__ 
status__closed closed
resolution__fixed fixed
component__core core
keywords__None None
time__1595108050971418 1595108050971418
changetime__1595116697844266 1595116697844266
version__None None
owner__twm twm

[twisted-trac]

	@twm set owner to @twm @twm set status to assigned

[twisted-trac]

	@twm removed owner @twm set status to new

PR here: #1357

[twisted-trac]

	@rodrigc set owner to @twm

Approved here: #1357 (review)

[twisted-trac]

	@twm set status to closed

In changeset f73824b

#!CommitTicketReference repository="" revision="f73824b62662fa91dbd334f707623456137a7a15"
Merge pull request #1357 from twisted/contributing-9916

Author: twm
Reviewer: rodrigc
Fixes: ticket:9916

Contributing documentation

[twisted-trac]

	@twm commented

Thanks for the quick review!