Add FESS_VERSION environment variable to README documentation

[marevol]

This update enhances the README by introducing the FESS_VERSION environment variable.

• Added export FESS_VERSION=15.1.0 in the setup instructions.
• Documented FESS_VERSION as a required environment variable for API compatibility, with default value 15.1.0.
• Updated example docker run commands to include FESS_VERSION.

This ensures users configure their environment correctly and avoid API mismatches with different Fess server versions.