Added note about ninja_syntax names being in the ninja namespace.

[PythonCHB]

Given all the confusion over on conda-forge, [*] I thought a little more docs would be helpful here.

If we had it to start all over again, I'd probably have simply installed ninja_syntax.py as a top level module, so it would work just like online docs suggest, e.g.:

import ninja_syntax
...
writer = ninja_syntax.Writer(sys.stdout)

from: https://bernsteinbear.com/blog/ninja-is-enough/

From a python-centric perspective this makes sense, too -- but should be documented :-).

The fact is that most (all?) folks using this package are simply getting it from a requirements file someone else set up -- I suspect there are very few people installing ninja from pypi so that they can use ninja_syntax anyway, but maybe some will?

I also added a note about:

python -m ninja

since it is there, it should be documented.

[*] conda-forge/ninja-feedstock#26