Wanted to jump in and confirm that everything pheonixstorm has said is correct. We are always looking for people to help out with the projects. One the main reasons to move the projects to Github was to make it easier for people to contribute to the projects.
Depending on code skills bug fixes, and implementing new rules are always appreciated. We also encourage people to ask for the main coders to have a look at their branches, especially if its going to be something big.
For people that are reading this going, it want to help but can't code. We are always looking for artistic types to make sprites (NOT mechs they are all done) but aerospace, and tanks. Want to make a Fire Mandrill Skin, play around and getting it looking decent and we can look at adding it. Canon Camo Schemes, we are also looking for Fluff images based on painted mini's. Adding summaries (not copied) to the unit files. Although not as important with some recent additions but RATs. If interested in helping with Data or art type work please PM me, or look for me in the Slack channel.
Also if you're not on the Slack channel for MegaMek I would encourage it. The various devs float in and out and can be tagged(summoned :) ) with the @ and their handle.