misc js lib update / minor fixes

* Minor JS lib updates (no apparent changes, just future proof code)

  * pnotify 1.20 -> 1.2.2
  * jquery.form 2.92 -> 3.35

* Add `CONTRIBUTING.md` to provide a guide for others.

* Corrected wrong config.ini, `NMJ_DBLOC` should be `NMJv2_DBLOC`. 

* Corrected config `usenet_retention` save routine value to match the init default