Add instructions for usage from npm

[bscan]

Currently, the "installation for other editors" is building from source. However, this is also hosted on npm: https://www.npmjs.com/package/perlnavigator-server

This should be installable using npm install. Ideally, we would also add an executable as well that calls the server in --stdio mode, so we could install globally with the -g flag and simply have it on our path.

[mcgaw]

I've added a bin entry to the server package. I came here to install PerlNavigator for the Helix editor. Since I'm on FreeBSD I had to install via NPM, and this change should make it immediately useable I think. #83