Changelog for cart documentation

[allyhawkins]

Before we release cart docs we are going to need a changelog entry. I'm filing this now to track anything that we might want to mention before we forget about it.
This includes:

• Introduction of 'My dataset'
• New download file structures
• New download options
• Change of single_cell_metadata.tsv to single-cell_metadata.tsv.

[allyhawkins]

Closed by #479