Clean up RELEASE.txt; adding 'tidy' instructions

Planning to run 'tidy' on the HTML doc files.  Documenting the
arguments I'm providing to 'tidy', for future reference,
within the RELEASE.txt file.

Also, cleaned up and improved RELEASE.txt.

docs/RELEASE.txt

@@ -1,8 +1,13 @@
 Release checklist for Tux Paint
+Last updated 2020-07-27 -bjk
 
-Places to make sure version # and release date gets updated, prior to
+Do these things _prior_ to cutting a release -- that is, prior to
-cutting a release (tagging in the source code repository, and running
+tagging in the source code repository, running "make release" to roll
-"make release" to roll a .tar.gz source tarball).
+a .tar.gz source tarball, and making the tarball available for download.
+
+ * Update version # and release dates
+
+   Places to make sure version # and release date gets updated,
 
    * Makefile
   
@@ -25,3 +30,16 @@ cutting a release (tagging in the source code repository, and running
    * Manual page (manpage)
      * src/manpage/tuxpaint.1
 
+ * Tidy the HTML documentation
+
+   Using 
+   (Be sure to run "make" in "docs/", to produce plaintext alternatives!)
+
+     tidy -i --markup yes --vertical-space yes --indent yes --indent-attributes yes INFILE.html > OUTFILE.html
+
+   Make certain it looks okay, and stil renders properly in a browser.
+   If not, correct the issues!  Then replace the original with the new
+   version 'tidy' created.
+
+ * Don't forget to commit the changes!  (git add ... / git commit / git push)
+