Clean up CONTRIBUTING: remove credits (outdated, mentioned elswehere) and remove info already in SECURITY.md

The list of main contributors was very outdated.

This marker is used by some Markdown compilers to start the Table of
Contents hierarchy at this point.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This section felt a little like a blob of good, unorganized thoughts. I
hope this is a little easier to read and understand. It will be easier
for someone to go to this document and point to a specific section now.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This comment is quite arbitrary in the view of the general public. Not
sure if anyone would know what the acronym "T.O.C." means.

The random line breaks everywhere in this file makes it hard to read at
narrow or wide window sizes whether you are using a Markdown reader or
just reading the markdown itself.

This puts all of the paragraphs and list items on their own line. The
one reading this file can decide for themselves how narrow or wide they
want the text to be displayed at.

I have also fixed the indentaiton of list items. Markdown will only read
a list item as "indented" when you use 4 spaces.

My final change in this commit was to remove all of the new lines below
headers. After reading so many markdown files, I find it much more
readable when there is one new line _before_ the header and zero new
lines below it.

It is much easier to review commits (e.g. with `tig`) if they have a
short subject line – e.g. a sysadmin might want to quickly skip all the
"docs: Foo" and "tests: Bar" commits and focus on the important parts.