chore(docs): add DISCLAIMER and update the copyright in NOTICE

[git-hulk]

See https://incubator.apache.org/policy/incubation.html#disclaimers

[spetz]

It does look like a duplicate of #1581

[Xuanwo]

It does look like a duplicate of #1581

Since #1581 has been closed, I think we can merge this one first.

[git-hulk]

Oh, I didn't notice that PR. I feel it should be good to update them separately with the following:

• Add DISCLAIMER and update notice since it's now under the Apache organization
• Add ASF license header for each source file
• Add all present third-party library license statements to the NOTICE

[spetz]

Ok, we will need to run the script to update each source file with the appropriate license header.

[caicancai]

I have no problem with this as it is a minor change