Add notes about maven extra configuration and gpg key.

[dmitry-shibanov]

Description:
In scope of this pull request we update documentation.

• Add notes about pom.xml configuration.
• Add notes about the GPG key import.
  Related issue:
• gpg: signing failed: Inappropriate ioctl for device #83
• Use GPG private key within GitHub Actions and Setup Java SDK #91
• Add a note in the readme how to export a gpg key #100

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[dmitry-shibanov]

Hello, everyone. Could you please take a look on this pull request. If you have any suggestion about docs improvement for maven and gpg.
cc: @galderz @davideicardi @bissim @bjansen

[bissim]

@dmitry-shibanov Looks OK to me as you did, concise and clear. 👌🏾