Add getting started guide for using pyODK in Google CoLab

[yanokwa]

I've run into a number of people who use Colab as their Python environment and struggle to get everything setup. Perhaps we could add some more step-by-step docs for these users?

I think the tricky bits are that they need to install pyodk and they need to persist their credentials. The below screenshot is how I set up things for myself and it very helpful to a customer. Couple of things to note

• I should have probably used ! instead of % to install pyodk
• Colab Notebooks is the default location that Colab saves notebooks to, so that's where I stored the credentials.
• The credentials are not hidden because how to edit hidden files require more explanation.

I'm not sure if this doc should live in pyODK or the docs site. @lognaturel, any thoughts on this?