Explain missing .mobi builds in readme

- Add new sentence explaining missing .mobi builds.
- Remove .mobi from list of output files
- Remove .mobi from bundle exec rake book:build example

Author: HonkingGoose

README.asc

@@ -15,7 +15,8 @@ See link:TRANSLATING.md[the translating document] for more information.
 == How To Generate the Book
 
 You can generate the e-book files manually with Asciidoctor.
-If you run the following you _may_ actually get HTML, Epub, Mobi and PDF output files:
+We used to be able to build .mobi files (Kindle), but cannot do so now, see #1496 for more information.
+If you run the following you _may_ actually get HTML, Epub and PDF output files:
 
 ----
 $ bundle install
@@ -24,8 +25,6 @@ Converting to HTML...
  -- HTML output at progit.html
 Converting to EPub...
  -- Epub output at progit.epub
-Converting to Mobi (kf8)...
- -- Mobi output at progit.mobi
 Converting to PDF...
  -- PDF output at progit.pdf
 ----