added a section that explains rebuilding sqlite3 for node-webkit

There is an engine called “[node-webkit](https://github.com/rogerwang/node-webkit)”. It allows to `require()` and use Node modules from the context of WebKit-based web browser's windows, and thus the methods of web development can be used to compose cross-platform GUI applications (for Windows, for Mac, for Linux) in JavaScript.

Quite fortunately, node-sqlite3 may run on both engines (Node.js and node-webkit) without any changes in the module's source code.

However, the ABI of Node.js and node-webkit are different, and thus the module has to be rebuilt (using nw-gyp instead of node-gyp) before it is used in node-webkit.

This pull request extends `README.md` and explains such a rebuilding.