Update README.md

Add `npm install` instruction at the _Running the CLI_ section.

README.md

@@ -52,7 +52,13 @@ If you are running this utility against a GHEC account, we recommend that you cr
 
 ## Running the CLI
 
-Execute the command using node or npm.
+Execute the command using node or npm. Before to execute, run `npm install` to install dependencies.
+
+```shell script
+$ git clone https://github.com/github/ghec-audit-log-cli
+$ cd ghec-audit-log-cli
+$ npm install
+```
 
 ### npm