Expand on core team workflow for adding / changing strings in `en` base pophelp

[petecooper]

We have a few changes that need to be made in the base pophelp files prior to 4.9.0. I am happy to cherry pick and process them as I can, but it's so long since I've done it that I don't want to eff it up.

Can we please have some clarification in README.md for core team (or nominated devs) to add strings to the base pophelp, please? That might simply be "add strings and tell @philwareham", but any reminder would be helpful.