README: Replace yarn with npm (because of #180) (#182)

Author: nikitastupin

README.md

@@ -21,7 +21,7 @@ Try it out at: http://graphql.org/swapi-graphql
 Install dependencies with
 
 ```sh
-yarn
+npm install
 ```
 
 ## SWAPI Wrapper
@@ -37,8 +37,7 @@ yarn test
 A local express server is in `./server`. It can be run with:
 
 ```sh
-yarn build # Only if you changed something
-yarn start
+npm start
 ```
 
 A GraphiQL instance will be opened at http://localhost:8080/ (or similar; the actual port number will be printed to the console) to explore the API.