Explain that GitHub PRs are temporarily disabled

This will need to be reverted when the issues migration is complete, but
wxTrac link will have to be removed/replaced.

.github/CONTRIBUTING.md

-# How to Contribute to wxWidgets
+# Temporary Notice
 
-We appreciate receiving contributions and we have a community-maintained
-[guide for contributors](https://wiki.wxwidgets.org/Development:_How_To_Contribute)
-which explains everything you ever wanted to know for contributing to
-wxWidgets.
+Pull requests to this repository are temporarily disabled to avoid interfering
+with the ongoing migration of our bug tracker to GitHub Issues.
 
-You can either submit your [patches](https://trac.wxwidgets.org/wiki/HowToSubmitPatches)
-to [wxTrac](https://trac.wxwidgets.org/newticket) or open GitHub pull requests,
-depending on your preferences.
+Please check back soon (the entire migration should take no more than 48
+hours).
 
-### Thanks in advance and looking forward to your contributions!
+### Sorry for the inconvenience!