Update license & copyright texts to mention contributors

LICENSE

@@ -1,4 +1,4 @@
-Copyright 2018, Sascha Cowley.
+Copyright 2018-2020, Sascha Cowley and contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy 
 of this software and associated documentation files (the "Software"), to 
@@ -16,4 +16,4 @@ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR 
 OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, 
 ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR 
-OTHER DEALINGS IN THE SOFTWARE.
+OTHER DEALINGS IN THE SOFTWARE.

README.md

@@ -44,7 +44,7 @@ MARKDOWN = {
 
 Thanks are owed to the author of [mdx_downheader][p2], whose code I examined for inspiration; and the contributers to the [default python-markdown extensions][pmdx], whose code I examined to get a better idea of what the [manual][pmdapi] was talking about.
 
-This project is copyright 2018 by Sascha Cowley under the [MIT License](LICENSE).
+This project is copyright 2018-2020 by Sascha Cowley and contributors [MIT License](LICENSE).
 
 
 [pmd]: https://python-markdown.github.io/
@@ -54,4 +54,4 @@ This project is copyright 2018 by Sascha Cowley under the [MIT License](LICENSE)
 [pmdx]: https://python-markdown.github.io/extensions/
 [pmdapi]: https://python-markdown.github.io/extensions/
 [pelicanconf]: http://docs.getpelican.com/en/stable/settings.html
-[3ppmdx]: https://github.com/Python-Markdown/markdown/wiki/third-party-extensions
+[3ppmdx]: https://github.com/Python-Markdown/markdown/wiki/third-party-extensions