updates README on installing Yarn
updates README on installing Yarn


Former-commit-id: 98824826dbc52a4560ca844c1e2356bbae8359d0
InnovateUKGitHub/innovation-funding-service,InnovateUKGitHub/innovation-funding-service,InnovateUKGitHub/innovation-funding-service,InnovateUKGitHub/innovation-funding-service,InnovateUKGitHub/innovation-funding-service