add instructions for populating dev db with accessibility data

[quoideneuf]

Adds a section describing how to populate the default dev database with some live data.

[mark-cooper]

@quoideneuf I didn't want to clobber your work so I created a separate branch to add some bits to it: https://github.com/archivesspace/tech-docs/pull/141

If you want to keep yours let me know and I can merge it, otherwise close this one and merge mine.

[quoideneuf]

👍