Add basic profiling info to readme

[psanford]

DonutDB is slower than a local SQLite file, but how much slower? It'd be nice to at least quantify this a little bit so people have a rough idea of the performance difference.

Maybe start with the queries listed here: https://www.sqlite.org/speed.html.

Since we use a global lock, its probably not worth profiling concurrent write workloads (donutdb those will all get serialized). I hope this is already clear in the readme, but its probably worth mentioning again alongside this data.