Restrict wiki modify access to main Jackson repo

[Tatu Saloranta]

Quick announcement: I changed write-access to Wiki pages on:

https://github.com/FasterXML/jackson

to only let collaborators make changes. This may be cumbersome wrt
contributing changes to Wiki pages -- which is unfortunate -- but
given that this repo is the entry point and public face of the whole
project, it seems necessary to reduce likelihood of anyone making
malicious changes.
At the same time, anyone who is interested in contributing content
should just contact me (or ask on the list) for access: it is at least
easy add users as collaborators. And maybe at some point in future
Github might offer PRs for Wiki too.

-+ Tatu +-