Adds documentation of emo::ji_find to README

[stephhazlitt]

I am loving adding emoji to my Rmd/md files, thank you! This PR offers a small addition to the README, adding the use of keywords and ji_find as a quick way to find the unique name of an emoji without leaving the console.

[norcalbiostat]

Can this be reviewed/pulled? I was looking for this info (i.e. how to find what the unique keyword is) and think it's necessary info. Especially since there is a detailed list of emoji with common keywords. [Edit] - this PR doesn't actually help with the unique keyword issue.