Add to features & known limitations

[ArBridgeman]

Summary

Features from 2.0

• caching provide SQLA
• auto-increment indexes

Known limitations

• foreign key
• custom constraints
• https://github.com/exasol/sqlalchemy-exasol/pull/707/changes#r2716084385

Other

• Double-check that ORM examples ok with +1 or want replaced with SQL queries -> @tkilias