Create architecture.md

Readd comments on architecture by Chris

docs/architecture.md

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+# Key parts
+
+# Updating the browser chrome on the page
+
+The web extension works by relying on on the background.js to make calls to the API, in response to listeners being triggered when a page loads, or tab is activated.
+
+In both cases `doGreencheckForTabReplace` is called, calling `getGreencheck` which then calls `doRequest` to update the chrome for the given tab, with the grey/green icon.
+
+# Updating the page DOM - links to green sites, and annotating search results
+
+In addition to updating the browser chrome, the extension also adds extra markup on the page on browsers, using the content scripts, `search-all.js`, `search-bing.js` and so on.
+
+These send messages to background.js with `runtime.sendMessage`, which triggers `chrome.runtime.onMessage` in background.js. This calls `doSearchRequest`, which triggers a batched API request with `doApiRequest`, to send info back to the tab, to update the DOM.
+
+More info on:
+https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webNavigation/onCommitted
+https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#Communicating_with_background_scripts
+https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendMessage