implement spell checking targets to aid tech writers and documentation contributors

- add two new targets to our Makefile: `spell' and `spell_all'
- we rely on `aspell' spell checking tool (http://aspell.net/), it must
  be installed in system
- `spell' target triggers aspell to check files that were touched by
  current commit; suggested workflow:
  * git checkout -b release-notes-vX.Y
  * edit files with your favourite editor
  * git commit -a
  * make spell
  * git commit --amend
  * git review
- `spell_all' target will run aspell against all .rst files found in
  pages/ directory
- provide aspell wordlist `.aspell_en.wordlist', so that aspell would
  not ask you each time when it finds unknown word or acronym (e.g.
  VLAN, PXE, DHCP, etc)
- mention `aspell' in README.md

Change-Id: I87a3dcdf5e3e926f737933a00c7597065e4f80ee
Merge "implement spell checking targets to aid tech writers and documentation contributors"