Add readme.html to top level .gitignore + sort it

The rationale behind it is that it's useful to sometimes use discount
(or rdiscount) to test that the README.md file formatting is correct but
readme.html is the obvious target file. This glob can be removed at a
later date.

Sort .gitignore file.

.gitignore

@@ -1,11 +1,12 @@
-.*.sw[a-z]
-.sw[a-z]
-.sv[a-z]
-.*.sv[a-z]
-*.dSYM
 *.bak
 *.bku
+*.dSYM
+.*.sv[a-z]
+.*.sw[a-z]
 .DS_Store
+.sv[a-z]
+.sw[a-z]
+/NOTES
 /archive/1984/
 /archive/1985/
 /archive/1986/
@@ -35,6 +36,6 @@
 /archive/2020/
 /bugs.html
 /faq.html
-mkentry
-/NOTES
 iocccsize
+mkentry
+readme.html