Clarify source code license

[bwitherspoon]

Originally this source code was under the Apache License:

https://github.com/openwch/ch32v307/blob/3116d1e37f39c7ee431e527222f2b9bb24a0e265/LICENSE

In response to #2 the license was removed and replaced with SPDX headers: 053ed57

This all seems to have been remove and replaced with a proprietary license with this update: 53f6e3d

Please clarify the license you are releasing this source code under and please add a LICENSE file and ideally SPDX file headers also so this is clear. I hope you will consider the Apache license again.