Missing LICENSE file

[jjscheel]

While the riscv-plic.adoc file references the project is licensed under the CC-BY-4.0 license and provides a link to the full license text. the lack of a LICENSE file prevent GitHub from recognizing the license.

I propose adding a plain text LICENSE file like the one used in the ISA manual.

I'm happy to add this into the project directly and will do so in the coming days. If anyone has concerns, please raise them.

Thanks,
-Jeff

[jjscheel]

Committed directly to master. Closing.