Merge #780

780: Mention node.js requirement in readme r=DJMcNab a=Matthias247

I tried building rust-analyzer according to the instructions, but it failed with a very non-descriptive error:

> will run: npm ci
> Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

It took me a while to figure out I had an outdated node version installed, which didn't support `npm ci`. I think mentioning the requirement explicitly might prevent others from running into the same issue.

Co-authored-by: Matthias Einwag <matthias.einwag@live.com>