[PATCH] Some documentation...

Hi

I'm starting to write some docs...
Comments... even "yep, looks OK, carry on" :)

I plan on putting the 'git command' ones into the 'git help ...' 
structure once Petr accepts it.
I guess the low level ones go into a README.reference until they 
stabilise and become man pages...

In doing this I noticed a couple of points:
* update-cache won't accept ./file or fred/./file
* checkout-cache doesn't seem to preserve mode

Are these bugs or should they be documented?
I've taken the approach of documenting behaviour for now.

Signed-off-by: David Greaves <redacted>
---