fix: add docs for node-gyp requirements

This is really "docs:" but this is supposed to trigger a release for the
node-gyp bump.

README.md

@@ -41,10 +41,10 @@ and then
 npm run rebuild
 ```
 
-### What versions of Node does it work with?
-
-Node v6.0.0 or higher is required.
+### What are the requirements?
 
+Node v6.0.0 or higher is required. Building the extensions use [`node-gyp`](https://github.com/nodejs/node-gyp#installation),
+refer to the link for its installation:runtime requirements.
 
 ### CLI Arguments