updated readme with more explicit API version info (#520)

README.md

@@ -18,11 +18,16 @@ The old `1.x.x` line of releases has been kept and is being published separately
 
 The actively developed line of releases is `2.x.x` and is contained within the `master` branch. New features are being added to this line of releases, so to develop this library further, create pull requests against the master branch.
 
+Below is a list of which Stripe API version recent releases of Stripity Stripe use. It only indicates the API version being called, not necessarily its compatibility. See the [Stripe API Upgrades page](https://stripe.com/docs/upgrades) for more details.
+
 `:stripity_stripe` | Stripe API Version
 ------------ | -------------
 `2.0.x` | `2018-02-28`
-`2.1.x - 2.2.x` | `2018-05-21`
-`master` | `2019-03-14`
+`2.1.0 - 2.2.0` | `2018-05-21`
+`2.2.2` | `2018-08-23`
+`2.2.3 - 2.3.0` | `2018-11-08`
+`2.4.0` | `2019-05-16`
+`master` | `2019-05-16`
 
 # Documentation