README: Update with instructions on how to build locally

[odscjames]

Overview

• What does this pull request do?

Update readme

• How can a reviewer test or examine your changes?

Read new section on building locally and test

• Who is best placed to review it?

Simon or Kadie

(Closes/Relates to) issue: #

Translations

• N/A I've notified the translation coordinator of any new strings that will need
  translating. See: https://openownership.github.io/bods-dev-handbook/translations.html

Documentation & Release

• N/A I've thought about how and when this needs to be released. See:
  https://openownership.github.io/bods-dev-handbook/standard_releases.html
• N/A I've updated the changelog, if necessary.
• N/A I've updated reference.rst to reflect any changes to schema structure or naming.
• N/A I've added or edited any other related documentation. See: https://github.com/openownership/bods-dev-handbook/blob/master/style_guide.md

[kd-ods]

Great! I've just tried it and it works like a dream.