Added RELEASE.txt blurb

Author: bdbaddog

RELEASE.txt

@@ -57,7 +57,8 @@ DOCUMENTATION
 DEVELOPMENT
 -----------
 
-- List visible changes in the way SCons is developed
+- runtest.py once again finds "external" tests, such as the tests for
+  tools in scons-contrib. An earlier rework had broken this.  Fixes #4699.
 
 Thanks to the following contributors listed below for their contributions to this release.
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