Add activity period to roles

[csarven]

Individuals are listed under roles but there is no indicator as to when they began or ended their work.

It's be great to include a column along the lines of an "active period" to be transparent as well as to ensure that contributors are credited (as opposed to just disappearing or being lost in the commit logs).

[melvincarvalho]

Is this issue related to this commit somehow?

https://github.com/solid/process/commit/f780386be4e4ca9f9b0770ec750c4d96b72e4c16