Update README.md (#2)

Be more detailed and explicit about the supported/unsupported features.

README.md

@@ -2,7 +2,7 @@
 [![Gem Version](https://badge.fury.io/rb/sorbet-coerce.svg)](https://badge.fury.io/rb/sorbet-coerce)
 [![Build Status](https://travis-ci.com/chanzuckerberg/sorbet-coerce.svg?branch=master)](https://travis-ci.com/chanzuckerberg/sorbet-coerce)
 
-Type coercion for [Sorbet](https://sorbet.org)
+A type coercion lib works with [Sorbet](https://sorbet.org)'s static type checker and type definitions; raises an error if the coercion fails.
 
 ## Installation
 1. Follow the steps [here](https://sorbet.org/docs/adopting) to set up the latest version of Sorbet and run `srb tc`.
@@ -26,7 +26,21 @@ converted = T::Coerce[<Type>].new.from(<value>)
 T.reveal_type(converted) # <Type>
 ```
 
-- Simple Type
+
+
+### Supported Types
+- Simple Types
+- Custom Types: If the values can be coerced by `.new`
+- `T.nilable(<supported type>)`
+- `T::Array[<supported type>]`
+- Subclasses of `T::Struct`
+
+We don't support
+- `T::Hash` (currently)
+- `T.any(<supported type>, ...)`: A union type other than `T.nilable`
+
+### Examples
+- Simple Types
 
 ```ruby
 T::Coerce[Date].new.from('2019-08-05')