[apache#4074] Add work in progress disclaimer. (apache#4076)

### What changes were proposed in this pull request?

Add work in progress disclaimer.

### Why are the changes needed?

Required by ASF incubator policy.

Fix: apache#4074

### Does this PR introduce _any_ user-facing change?

N/A

### How was this patch tested?

N/A

DISCLAIMER_WIP.txt

@@ -0,0 +1,6 @@
+Apache Gravitino is an effort undergoing incubation at The Apache Software Foundation (ASF), sponsored by the name of Apache TLP sponsor. Incubation is required of all newly accepted projects until a further review indicates that the infrastructure, communications, and decision-making process have stabilized in a manner consistent with other successful ASF projects. While incubation status is not necessarily a reflection of the completeness or stability of the code, it does indicate that the project has yet to be fully endorsed by the ASF.
+
+Some of the incubating project’s releases may not be fully compliant with ASF policy and while we have documented the licensing of all code in detail, we know that currently our release would:
+- Contains code that may not be compatible with the Apache License
+
+If you are planning to incorporate this work into your product/project, please be aware that you will need to conduct a thorough licensing review to determine the overall implications of including this work. For the current status of this project through the Apache Incubator, visit: https://incubator.apache.org/projects/Apache Podling-Name.html

build.gradle.kts

@@ -484,6 +484,8 @@ tasks.rat {
     "web/src/lib/icons/svg/**/*.svg",
     "**/LICENSE.*",
     "**/NOTICE.*",
+    "DISCLAIMER_WIP.txt",
+    "DISCLAIMER.txt",
     "ROADMAP.md",
     "clients/client-python/.pytest_cache/*",
     "clients/client-python/gravitino.egg-info/*",