Docs: adjust INSTALL to reflect 2.5.1 behaviour
Docs: typo
Docs: turn these into local refs not full URLs
Docs: typo
Requirements: min and pinned are generated files.

Clarify that minimum and pinned requirements are generated files.

I think I finally understand the need for these files - sorry Alex

- min-requirements.txt - these are the minimum versions in requirements/*.txt.
  Call this the minimum that we contractually agree will work.
- requirements.txt - or the pinned version.  Pinned because at some time in
  space we pinned it to these versions.  Essentially these are the maximum
  versions that were available at the time of release.  After we release
  versions will increase and their is a risk that a neweer version won't work
  with Pootle.  The pinned requirements is the contractual maximum that should
  work.

What is missing is that we don't actively test against these versions.