Add STI description to README
typo

Misspelled "library"
Merge pull request #81 from vinbarnes/patch-1

fix typo
change error when specifying multiple attributes
Merge pull request #82 from vinbarnes/update-error-message-for-missing-column

clarify missing field error message when using an Array for the with_same option
Merge pull request #78 from chrisdpeters/master

Add STI description to README