Chore: add npm scripts

[JPeer264]

Actually you have done everything right in #5 (comment). I just assume you have called the scripts wrongly. Once you have set up your npm scripts, you also have to call them as such. In this PR I introduced the scripts watch and build. These should be executed as npm run watch or npm run build (as I mentioned in the Dev Setup).

Just a hint, if you want to test that gulp is running in your npm scripts, you can write ./node_modules/.bin/gulp into your shell. As in ./node_modules/.bin are all modules which are executable within your npm script (hope this was not confusing).

Here I added a screenshot to illustrate it. In the first section I ran gulp, but gulp is not installed globally - so the command is unknown. In the second part, I ran gulp via the local node_modules. And in the third part, I run everything in the npm scripts, where nothing is installed globally now. But this does not mean you can't have a globally version of gulp as well 👍

Btw, within your Dev Docs I changed the path in (now) point 5. I just added a ./ before, so it is obvious that it is a directory in the root.

[ajusa]

I'll take a look at this tomorrow