Improve docs

[danyill]

Something I'd like to do:

• Mention API docs at https://openenergytools.github.io/scl-lib/
• Provide instructions on installation
• Provide a few examples:
  1. Subscribing an ExtRef to a DataSet
  2. Renaming an IED
  3. Find all subscriptions to a control block
• The examples should most likely be complete, showing how to dispatch an event and update an XML document.