Add technical notes.

docs/devnotes.md

@@ -0,0 +1,13 @@
+# Technical Notes
+
+## Frontend
+
+- using [preact](https://github.com/developit/preact) for a smaller setup that 
+    is compatible with React.
+- [importing preact into modules](https://github.com/developit/preact#import-what-you-need)
+- using [preact-compat](https://www.npmjs.com/package/preact-compat) to avoid 
+    migrating all of React code to preact.
+- [server side rendering in preact](http://thecodebarbarian.com/server-side-rendering-with-preact-and-firebase.html)
+- use `process.env.MY_FLAG' using 
+  [webpack's DefinePlugin to pass in envs](https://webpack.js.org/plugins/define-plugin/#feature-flags).
+    - this is useful when dealing with separating server-only and client-only code.