Document our LLVM dependency and steps to upgrade
Document our LLVM dependency and steps to upgrade

Bug: b/165000222
Change-Id: I0044dcb095ae1ae195de153485088f91fa987fb6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/61828
Reviewed-by: Sean Risser <2d04e98eeaa433fb55534399ab3ab2c06fdf7967@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

google/swiftshader,google/swiftshader,google/swiftshader