Mention installing html5lib to run the tests in the readme

[DavidBruant]

Arriving at python tools/scripts/manifest.py MANIFEST.json, I met an error:

Traceback (most recent call last):
  File "tools/scripts/manifest.py", line 466, in <module>
    update_manifest(repo_root, out_dir)
  File "tools/scripts/manifest.py", line 456, in update_manifest
    update(manifest)
  File "tools/scripts/manifest.py", line 436, in update
    import html5lib
ImportError: No module named html5lib

[DavidBruant]

And I guess installing pip as well?

I forgot to mention I'm on the latest Ubuntu stable and don't really have experience with Python nor more environment than provided by default.

[DavidBruant]

I personally solved the issue doing

sudo apt-get install pip-python
sudo pip install html5lib