added bolding of "must have"

docs/source/whatsnew/pr/README.md

@@ -1,18 +1,18 b''
 Documenting What's New
 ----------------------
 
 When making a new pull request that either adds a new feature, or makes a
 backwards-incompatible change to IPython, please add a new `.rst` file in this
 directory documenting this change as a part of your Pull Request.
 
 This will allow multiple Pull Requests to do the same without conflicting with
 one another. Periodically, IPython developers with commit rights will run a
 script and populate [development.rst](../development.rst)
 with the contents of this directory, and clean it up.
 
 Files which describe new features can have any name, such as
-`antigravity-feature.rst`, whereas backwards incompatible changes should have a
-filename starting with `incompat-`, such as `incompat-switching-to-perl.rst`.
-Our "What's new" files always have two sections,  and this prefix scheme will
-make sure that the backwards incompatible changes get routed to their proper
-section.
+`antigravity-feature.rst`, whereas backwards incompatible changes **must have**
+have a filename starting with `incompat-`, such as
+`incompat-switching-to-perl.rst`.  Our "What's new" files always have two
+sections,  and this prefix scheme will make sure that the backwards incompatible
+changes get routed to their proper section.