update readme w/ install instructions

Author: brockelmore

README.md

@@ -2,6 +2,12 @@
 
 Forge Standard Library is a collection of helpful contracts for use with [`forge` and `foundry`](https://github.com/gakonst/foundry). It leverages `forge`'s cheatcodes to make writing tests easier and faster, while improving the UX of cheatcodes. For more in-depth usage examples checkout the [tests](https://github.com/brockelmore/forge-std/blob/master/src/test).
 
+## Install
+
+```bash
+forge install brockelmore/forge-std
+```
+
 ## Contracts
 ### stdError
 
@@ -229,4 +235,4 @@ contract Bar {
         require(msg.sender == expectedSender, "!prank");
     }
 }
-```
+```