Add license

[sdf-jkl]

This should be it...

• closes Pick licensing #15

[friendlymatthew]

Thanks @sdf-jkl

I think it is recommended to place a short license header as a comment at the top of each source file, but I don't feel too strongly about it

Lmk if you feel otherwise, if not, I'll plan on merging

[sdf-jkl]

@friendlymatthew

Wording here makes it sound like headers are needed to make specific files use the license if the whole project is not using it.

This mentions that headers and NOTICE file are required for projects submitted directly to ASF.

Overall, we don't have to include headers and NOTICE file until this repo goes upstream.

We can merge.

[friendlymatthew]

Thanks again @sdf-jkl