Improve direnv instructions regarding mandatory use
Improve direnv instructions regarding mandatory use

Currently our Makefiles require an .envrc file to exist for each
project so the instructions should reflect that.

Note, technically you only need to have the file exist, it does
not actually need to export required environment variables - you
could still do that manually through the command line if you
wished. In reality though, I think most people will be happy
using direnv for this purpose.

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws