Fix and expand repo instructions

[veggiesaurus]

• Additional python dependencies (plantweb and sphinx_rtd_theme) are required in order to build the docs.
• The readme needs to be updated wiht a checklist for creating a PR:
  • How to pick a new version number
  • Where to update the version
  • How to regenerate and check the docs
  • Some basic checklist items for major revisions: Have the rest of the docs been updated (e.g. the Behaviour section), with sequence diagrams if necessary.