Better explain the formatting and length of shared keys

[cpswan]

Is your feature request related to a problem? Please describe.

When we create an AES shared key it will be 256 bits long (== 32 bytes), but the shared key that we find in an atSign is much longer, as it's encrypted with RSA2048, which pushes its size up to 2048 bits (== 256 bytes) before it is then base64 encoded, which takes the total size to around 350 characters

Describe the solution you'd like

Add to the documentation in this repo so that expectations around keys are made clear.

[gkc]

See also #46

I've covered this in a larger update to the protocol spec for which I will be raising a PR this week. Keeping in sprint but reducing SP to 0

[gkc]

This didn't get attention during last two sprints; will prioritize closing this out this week