StackOverflow has merged friendly_id and friendly-id tags.

No need to mention friendly_id tag anymore since SO will automatically convert friendly_id to friendly-id at this point and SO convention prefers friendly-id anyway.
Merge pull request #468 from sporkmonger/patch-1

Updated README. StackOverflow has merged friendly_id and friendly-id tags.

[ci skip]
Added note about Active Admin

Resolves #441
Prepare for 5.0.0 release