chore(release): update readme for supported releases (#172)

Update supported versions of Dgraph in README.

Author: joshua-goldstein

README.md

@@ -42,7 +42,9 @@ use a different version of this client and their corresponding import paths.
 |  dgraph 20.03.0    |  dgo 200.03.0 | "github.com/dgraph-io/dgo/v200" |
 |  dgraph 20.07.0    |  dgo 200.03.0 | "github.com/dgraph-io/dgo/v200" |
 |  dgraph 20.11.0    |  dgo 200.03.0 | "github.com/dgraph-io/dgo/v200" |
-|  dgraph >= 21.03.0 |  dgo 210.03.0 | "github.com/dgraph-io/dgo/v210" |
+|  dgraph 21.X.Y     |  dgo 210.X.Y  | "github.com/dgraph-io/dgo/v210" |
+|  dgraph 22.X.Y     |  dgo 210.X.Y  | "github.com/dgraph-io/dgo/v210" |
+|  dgraph 23.X.Y     |  dgo 230.X.Y  | "github.com/dgraph-io/dgo/v230" |
 
 Note: One of the most important API breakages from dgo v1 to v2 is in
 the function `dgo.Txn.Mutate`. This function returns an `*api.Assigned`