This commit refactors the codebase to consistently use Python 3.6
f-strings for string concatenation and interpolation.

BREAKING CHANGE: This update increases the minimum required Python
version from 3.5 to 3.6. Python 3.5 reached end-of-life in September
2020 (~4 years ago) and has been dropped from most distributions, so the
impact on users should be minimal.

This is what is imported at the beginning of the examples.
This could also be done by changing the import to 'import mwclient'
rather than 'from mwclient import Site'.

Neither seems more correct but I have tried to follow
what seems most intended.

>>> from mwclient import Site
>>> site = mwclient.Site('en.wikipedia.org')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'mwclient' is not defined

vs.

>>> site = Site('en.wikipedia.org')
>>> page = site.pages['Greater guinea pig']
>>> text = page.text()
>>> page.exists
True

- Adds link to the user guide from readme
- Adds a new page to the user guide on "working with files"