Update - bash instructions for last_modified key in posts yaml

[lwasser]

i created a small bash script that parses through git history and updated the last_modified key in our _posts dir if it exists.

this script is NOT in ci yet (but should be eventually) and you have to know about it to run it locally. It would be good to document how this works in our readme file.

[lwasser]

this has been done -- see here