Trac #29640: Create an AUTHORS.md file with links

From README.md:

  The Sage Library is GPLv2+, and included packages have [compatible OSS
  licenses](./COPYING.txt). [Over 400 people](https://www.sagemath.org
/development-map.html)
  have contributed code to Sage. In many cases, documentation for
modules
  and functions list the authors.

In this ticket, we add an AUTHORS.md file, which will contain
this link too, and thus lead to the several more sources
where developers' contributions to Sage can be seen (including
maintenance of spkgs - see #29636):

 - Changelogs
 - some Trac searches
 - ...

https://doc.sagemath.org/html/en/faq/faq-general.html#who-is-behind-
this-project also has similar links.

URL: https://trac.sagemath.org/29640
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

AUTHORS.md

@@ -0,0 +1,9 @@
+# Authors of SageMath #
+
+[Over 800 people](https://www.sagemath.org/development-map.html) have
+contributed code to the Sage library (see [src/](src/)) or have
+contributed to the maintenance of the SageMath distribution (see
+[build/](build/)).
+
+See https://www.sagemath.org/development-map.html for information
+about the developers and their contributions.