Add description to manifest.json
Add description to manifest.json

As recommended by the [manifest docs][]. This shows up in
chrome://extensions and, I presume, in the Web Store when it's uploaded.

[manifest docs]: https://developer.chrome.com/extensions/manifest

freerange/freeagent_transaction_explainer,freerange/freeagent_transaction_explainer,freerange/freeagent_transaction_explainer