Automatically respond to new user actions

[RayStick]

In other GitHub projects I've been involved in, they provide welcome messages in the form of comments when

1. someone opens their first Issue
2. someone opens their first PR
3. someone merges their first PR

The welcome message is a nice friendly touch but can also provide an opportunity for them to be automatically reminded of where to look for relevant resources and what checklist to follow. This is done via a .github/config.yml file using this bot - https://github.com/apps/welcome

Steps to take:

• Implement the 3 welcome/congrats messages in this test repo
  • It works! See this issue and this PR for a working example
  • Fix image rendering
• Implement the 3 welcome/congrats messages in phys2bids
• Implement the 3 welcome/congrats messages in other repos

References #physiopy/phys2bids#316 which wants to produce a checklist for the main reviewer of a PR