Fix how to install (#86)

* fix how to install

I updated the readme of mockgen to reference the latest package, as it was previously referring to an outdated one.
Additionally, I have added instructions on how to install gomockhandler itself.

* Update README.md

Co-authored-by: Kensei Nakada <handbomusic@gmail.com>

---------

Co-authored-by: Kensei Nakada <handbomusic@gmail.com>

README.md

@@ -36,6 +36,12 @@ You have to install `mockgen` first.
 go install go.uber.org/mock/mockgen@latest
 ```
 
+Next, install gomockhandler.
+
+```
+go install github.com/sanposhiho/gomockhandler@latest
+```
+
 ## How to use
 
 `gomockhandler` is designed to be **simple** and does only three things.