Write a meaningful “conribution” section in the readme

[kirelagin]

Write something useful, based on @ixahedron’s experience contributing to this project, since there are plenty non-obvious things, e.g.:

• What goes to NaCl, what goes to crypto-sodium.
• When to use sized types.
• Where to lookup constants.
• When and how to get rid of IO.

[kirelagin]

• How to choose whether to keep a bytearray-like type polymorphic or to fix a specific type.