Build instructions are missing a step

Attempting to build capacitor-mapbox as detailed in the instructions leads to the following error:
```
> capacitor-mapbox@0.0.1 build
> npm run clean && tsc

> capacitor-mapbox@0.0.1 clean
> rimraf ./dist

sh: /Users/idosius/IdeaProjects/demo-capacitor-mapbox/capacitor-mapbox/node_modules/.bin/rimraf: Permission denied
npm ERR! code 126
npm ERR! path /Users/idosius/IdeaProjects/demo-capacitor-mapbox/capacitor-mapbox
npm ERR! command failed
npm ERR! command sh -c rimraf ./dist

npm ERR! A complete log of this run can be found in:
npm ERR! /Users/idosius/.npm/_logs/2021-03-16T09_38_36_456Z-debug.log
npm ERR! code 126
npm ERR! path /Users/idosius/IdeaProjects/demo-capacitor-mapbox/capacitor-mapbox
npm ERR! command failed
npm ERR! command sh -c npm run clean && tsc

npm ERR! A complete log of this run can be found in:
npm ERR! /Users/idosius/.npm/_logs/2021-03-16T09_38_36_513Z-debug.log
```

One step is missing before the build: running `npm install` in the project root directory