Document the production-readiness of python-swiftclient as a library

[wolever]

After a few months of using the python-swiftclient library in production, it's very clear that this library is not suited to production environments.

Specifically, the lack of:
- connection pooling
- auth token caching (especially between threads and processes)
- thread safety

Make it quite challenging to use in high-performance, concurrent environments.

It would be nice if these limitations were explicitly documented so other folk can avoid the headaches I've gone through working around these limitations.