Add line to install Node.js

For someone who isn't used to building fe things like this, it's
not always clear to install Node.js or what version.

A line has been added to the installation instructions pointing to
resources where to get it and what version to use. For version I
point to the woodpecker config because that is what the CI uses and
therefor always "tested".

There was a file .node-version who contained a node version, but
this was seriously outdated and removing it didn't seem to break
anything. Assuming it indeed doesn't do anything any more, it seems
better to remove to avoid confusion.

.node-version

@@ -1 +0,0 @@
-7.2.1

README.md

@@ -20,6 +20,8 @@ To use Akkoma-FE in Akkoma, use the [frontend](https://docs.akkoma.dev/stable/ad
 
 ## Build Setup
 
+Make sure you have [Node.js](https://nodejs.org/) installed. You can check `/.woodpecker.yml` for which node version the Akkoma CI currently uses.
+
 ``` bash
 # install dependencies
 corepack enable