Add note about how to add your own customization

Closes #146
Provide a useful binding of C-c C-z in sql-mode
Use highlight-symbol-mode in html mode too
Save more variables in desktop session file
Don't override M-? when paredit is active
Make C-x C-e run 'eval-region if the region is active
Make rename-this-file-and-buffer work when file is still unsaved
In browse-current-file, complain if the file is remote
Add sanityinc/ namespace to some utility functions
Don't ever highlight trailing whitespace in the minibuffer
Suggest the obvious customisation alternative of forking the repo
Bind smex to M-x by remapping execute-extended-command
Merge branch 'master' of github.com:purcell/emacs.d