Update installation instructions in README.md (#26)

This minimal PR documents the installation command. It took me a moment
to look it up from the github.com/golang repo. Hopefully this can help
save a few moments for others.

---------

Co-authored-by: Sung Yoon Whang <sungyoonwhang@gmail.com>

Author: SellJamHere

README.md

@@ -22,6 +22,14 @@ go.uber.org/mock supports all Go versions supported by the official
 [Go Release Policy](https://go.dev/doc/devel/release#policy). That is,
 the two most recent releases of Go.
 
+## Installation
+
+Install the `mockgen` tool.
+
+```
+go install go.uber.org/mock/mockgen@latest
+```
+
 ## Running mockgen
 
 `mockgen` has two modes of operation: source and reflect.