chore: Mention bundled FlatBuffers in LICENSE.txt

[kou]

What's Changed

It's not required but it's better to mention it:

https://infra.apache.org/licensing-howto.html#alv2-dep

Bundling an Apache 2-0-licensed dependency

Assuming that the bundled dependency itself contains no bundled sub-components under other licenses, so the ALv2 applies uniformly to all files, there is no need to modify LICENSE. However, for completeness it is useful to list the products and their versions, as is done for products under other licenses.

If the dependency supplies a NOTICE file, its contents must be analyzed and the relevant portions bubbled up into the top-level NOTICE file.

Closes #22.