fix: updates doco to include missing command (#268)

Signed-off-by: Anthony D. Mays <[email protected]>

Author: anthonydmays

lesson_06/README.md

@@ -23,6 +23,7 @@ HW_VERSION=D
 ```
 2. Run the program to determine the expression you must implement.
 ```bash
+npm install
 npm run compile
 npm start
 ```