[COMMUNITY] Add DISCLAIMER, KEYS for ASF release (#4345)

* [COMMUNITY] Add DISCLAIMER, KEYS for ASF release

* Add file name spec

DISCLAIMER

+Apache TVM (incubating) is an effort undergoing incubation at The
+Apache Software Foundation (ASF), sponsored by the Apache Incubator PMC.
+
+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.

KEYS

+This file contains the PGP keys of various developers.
+Please don't use them for email unless you have to. Their main
+purpose is code signing.
+
+Examples of importing this file in your keystore:
+ gpg --import KEYS.txt
+ (need pgp and other examples here)
+
+Examples of adding your key to this file:
+ pgp -kxa <your name> and append it to this file.
+ (pgpk -ll <your name> && pgpk -xa <your name>) >> this file.
+ (gpg --list-sigs <your name>
+     && gpg --armor --export <your name>) >> this file.
+
+-----------------------------------------------------------------------------------
+pub   rsa4096 2019-11-15 [SC]
+      EF52D68AD5276994249816836754EA97C55E3DEB
+uid           [ultimate] Tianqi Chen (CODE SIGNING KEY) <tqchen@apache.org>
+sig 3        6754EA97C55E3DEB 2019-11-15  Tianqi Chen (CODE SIGNING KEY) <tqchen@apache.org>
+sub   rsa4096 2019-11-15 [E]
+sig          6754EA97C55E3DEB 2019-11-15  Tianqi Chen (CODE SIGNING KEY) <tqchen@apache.org>
+
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+
+mQINBF3OK24BEADD4hxjrsgb4jIDIACHS15X+5YP/YaUF5UDDQs/bNn/xGJGVl4/
+4sJ6qKZcvMDrWTmnNItYBuaHi1qhGvlcASBekm/9PU2U8lZmAF1lZkKIIYZkX+If
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+ha8fS9ysWDy/Ye65MP4=
+=MSiP
+-----END PGP PUBLIC KEY BLOCK-----

NOTICE

-TVM End to End Deep Learning Compiler Stack: https://tvm.ai/
+Apache TVM (incubating)
+Copyright 2017 and onwards The Apache Software Foundation
+
+This product includes software developed at
+The Apache Software Foundation (http://www.apache.org/).
+

tests/lint/check_file_type.py

@@ -95,6 +95,8 @@ ALLOW_SPECIFIC_FILE = {
     "docker/with_the_same_user",
     "LICENSE",
     "NOTICE",
+    "KEYS",
+    "DISCLAIMER",
     "Jenkinsfile",
     # sgx file
     "apps/sgx/enclave/sgx-deps.diff",