[minor improvement] clarify Developer name requirement

[html5cat]

During login, CLI asks for Developer name – would be nice to have clarification about how it's used and what the requirements are before entering it.

[giovannibenussi]

I just had the same issue. I tried to use "Giovanni" and an error saying "Validation error" was raised.
However, using "giovanni" (with only lowercase letters) solved the problem, but was just trial and error.

[katsar0v]

This stuff should be documented, that's really not intuitive! Example:

Enter your Shoutem credentials (obtained at https://builder.shoutem.com):
? Email app@*********
? Password ********************
Enter developer name.
? Developer name Katsarov
Validation error

Use shoutem last-error for more info

Then:

Enter your Shoutem credentials (obtained at https://builder.shoutem.com):
? Email app@*********************
? Password ********************
Enter developer name.
? Developer name kristiyan
Registered as `kristiyan`.