Release note README link update (#1449)

The current link in the README directs to a github page that has a link
to the documentation page on how to write release notes. I've updated
the link here to just go straight to the documenation page on how to
write good release notes.

upcoming-release-notes/1449.md

+---
+category: Maintenance
+authors: [shall0pass]
+---
+
+Update link in README for release notes
\ No newline at end of file

upcoming-release-notes/README.md

-See the [Writing Good Release Notes](https://github.com/actualbudget/docs#writing-good-release-notes) section of the README for the documentation repo for more information on how to create a release notes file here.
+See the [Writing Good Release Notes](https://actualbudget.org/docs/contributing/#writing-good-release-notes) section of the README for the documentation repo for more information on how to create a release notes file here.