README.md libmincrypt URL broken (NOT_FOUND)

[orogenic]

As it's a link to the master branch, it's not a stable link. I think you can use a more stable link such as https://android.googlesource.com/platform/system/core/+/refs/tags/android-7.0.0_r29/libmincrypt/ (or whatever tag is relevant to when you wrote this document).

[cstyan]

Good idea, PRs are welcome :)