put Composer instructions with package name in the README

[joachim-n]

The README doesn't give a command line snippet for installing this. Which means that developers will look to the github URL for the package name to copy.

Unfortunately, there is a mismatch:

• localgovdrupal/localgov_project -- github URL
• localgovdrupal/localgov-project -- composer.json package name

It would be helpful to add standard installation instructions to the top of the README.