docs: document creation of __reana schema

[diegodelemos]

When deploying REANA on an external DB system admins might not have permissions to create the __reana schema

Possible solutions to be documented

• 1. Grant schema creation privilege

$ pgcli -h reana.external.db -p 6666 -U admin reana
reanaqa> GRANT CREATE ON DATABASE reana TO reana;

• 2. Create schema manually with admin user

$ pgcli -h reana.external.db -p 6666 -U admin reana
reanaqa> CREATE SCHEMA __reana
reanaqa> GRANT ALL ON SCHEMA __reana TO postgres;
reanaqa> GRANT ALL ON SCHEMA __reana TO public;

Stems from reanahub/reana-db#81.