Add docs for running litd in regtest in a Polar environment

[otech47]

Seems like it may be possible (https://github.com/lightninglabs/lightning-terminal/issues/359#issuecomment-1207956072) currently to run litd on regtest (with any risks?) but at least for development purposes it would be great if the README or some other doc detailed how to get this running

I tried to get it working with Polar and blocked by some issue with the macaroon service. Not 100% sure if this can be solved from litd or maybe something specific to Polar... leaving this here in case it is helpful or if anyone can solve it

Getting error error starting macaroon service: unable to unlock macaroon DB: invalid password

with specs:

OSX 12.6 (Apple M1 ARM) Polar 1.4.0 Docker 4.13.0 LND (tried with custom LND , and v15.2) LitD (v0.8.2-alpha commit)