Plugins also only need to be written once for both the 32-bit and 64-bit versions of Construct - the V8 engine handles the differences itself.
Location of plugins
Plugins are stored under <install path>\exporters\html5\plugins. Each plugin has its own folder, with four files:
edittime.js - defines everything the editor needs to know about the plugin.
runtime.js - defines the plugin methods that run in the browser.
common.js - prepended to both edittime.js and runtime.js, in case there's any code that is in common to both files. No plugins currently use this, it was added in anticipation of code duplication.
PluginIcon.ico - icon file loaded by the editor to represent the plugin.
To make a plugin, simply copy and paste another plugin's folder and start editing the files. (There's no template yet - sorry!)
There is no SDK documentation right now, mainly because there's just no time to get it all written down. Also, the SDK is subject to change at any time! If you write a plugin now, be prepared to have to change it to fix it after some releases. This is due to the nature of pre-alpha software: it will change a lot, and it will break things.
common_prelude.js - prepended to both edittime scripts and runtime scripts.
edittime_prelude.js - prepended only to edittime scripts.
preview_prelude.js - prepended only to runtime scripts.
It is not necessary to minify/obfuscate your scripts. Construct 2 gives the option to minify script on export.
Hope that helps get you started! Just be warned, the SDK may change at any time as functionality is added and changed.