Anywhere we can use `blob/master` we can use `blob/HEAD` instead. This
will make life easier if we ever rename our default branch in future
(once/if Git and GitHub provides the necessary tooling to do so).

This allows maintaining more settings in one place.

Mostly edits to formatting, grammar, wording for consistency & readability.

These are nicer on the eyes.

Capitalized beginning of each bullet point to match style of README.md

fixed link url to navigate to main brew repo instead of forked repo

May as well use the more secure link when possible.

- Note issues may be closed without comment if sufficient information is
  not provided.
- Deprecate Homebrew/brew-evolution, note new feature request flow
- Note some issues should be created on Homebrew/brew

Indicate that feature requests should go there rather than here.

The current one returns a 404.

Closes Homebrew/homebrew#48857.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

To avoid extra work on the part of contributors and maintainers, this
adds a step to the explanation of how to contribute a fix to a formula,
showing how to check if someone else has already done so, even if it
has not yet finished being tested, reviewed, and released.

Closes Homebrew/homebrew#48879.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Remove duplication, link to the API documentation more often,
tweak wording, add `@UniqMartin` as a maintainer, note `@jacknagel`'s
and `@adamv`'s significant past contributions to Homebrew, delete some
outdated or unneeded documentation, add some missing `Formula` API.
entries and simplify/improve `CONTRIBUTING.md`.

Closes Homebrew/homebrew#46179.

Closes Homebrew/homebrew#46618.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Feature it a bit more prominently in the documentation as a result.

Closes Homebrew/homebrew#45374.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Closes Homebrew/homebrew#33211.

Add the Homebrew Code of Conduct and Diversity statement.

These are heavily based on those created by Python:
http://www.python.org/psf/codeofconduct/
http://www.python.org/community/diversity/

Closes Homebrew/homebrew#26904.

To match the recent renaming of that wiki article.

Closes Homebrew/homebrew#17249.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>