docs: add DISCLAIMER and NOTICE

[BeiBaiNian]

As an Apache (Incubating) project, the repository root is missing the two files required by the Apache Incubator:

• DISCLAIMER: states that the project is undergoing incubation at the ASF and has not yet been fully endorsed by the ASF
• NOTICE: declares copyright ownership as required by the ASF, covering both The Apache Software Foundation copyright and the original Casbin project copyright

Verified on 2026-08-06 that neither file exists in upstream, fork, or local (all at 8134e41), and that Apache Casbin is still incubating (entered incubation 2026-02-07, not graduated). Only 4 of 239 apache/casbin-* repos have these files so far (casbin, jcasbin, jcasbin-jdbc-adapter, pycasbin).

What changed:

• Add DISCLAIMER (5 lines) - standard ASF incubation disclaimer, byte-identical to casbin / casbin-pycasbin / casbin-jcasbin-jdbc-adapter
• Add NOTICE (8 lines) - Copyright 2020-2026 The casbin Authors. (2020 matches this repo's first commit on 2020-02-10)

Verification:

• DISCLAIMER blob identical to casbin-pycasbin (552 bytes)
• NOTICE contains "Apache Casbin (Incubating)" / "Copyright 2026 The Apache Software Foundation" / "Copyright 2020-2026 The casbin Authors."
• Commit ef9df66 contains only NOTICE + DISCLAIMER (13 insertions), working tree clean