Added proposal requirements to vip-go-mu-plugins PR template

As part of improving our SDLC process, we’re going to require proposals for feature additions / significant feature alterations. This will allow us to ensure that features and significant feature alterations deliver much more value to our platform by being properly planned out, executed, and released.

https://github.com/Automattic/vip-go-mu-plugins/pull/1908

#mu-plugins