Add guidelines explaining what attachments (and contexts) should look like

[TimDiekmann]

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

We should add a section in the crate-level documentation on how to write attachments and context messages.

Describe the solution you'd like

We probably want to suggest using non-capitalized messages, this aligns well with the Rust API guidelines.

Describe alternatives you've considered

Capitalizing attachments may be more readable, however, it's a common practice to put errors in context and errors should not be capitalized. This results in inconsistency.