Upon @jp request.

The link is absolute URL, since the readme can be shown in a lot of
places (on lab.nexedi.com, on pypi, on github, etc...)

We might also change the link to demo_zbigarray.py to be absolute URL
because of pypi case in the future.

In the time when README.rst was initially introduced (58279ac7 "readme:
Initial draft") GitLab was not handling relative links well and it was
necessary to construct them properly by hand.

Nowdays GitLab automatically adds namespace and project and blob/ref/
prefix to relative URL link, so there is no need to specify them
manually.

Actually specifying the prefix manually makes URL invalid, as it becomes
something like

    https://lab.nexedi.com/nexedi/wendelin.core/blob/master/wendelin.core/blob/master/demo/demo_zbigarray.py

Fix it.

This is a general readme about what wendelin.core is and how to use
ZBigArray from user point of view.