Update PRMI_playground/README.md

See merge request IFOsim/aundhaisc!1

generating files with different internal finesses up to maxtem 4. Plotting how the resulting error signals degrade with misalignment (currently SRCL with SR3 xbeta). Attempts to parallelise the file generator since it's clunky and slow

Auto-setting PRM,SRM to match aLIGO QNLS for diff ITM transmissions, plotting LSC errsigs. PRM-setter needs change, currently gives sudden jump in results

 adding notes files from todays call, forgot to pull ReadMe modifications first...