readme: add mapping between llir/llvm v0.3.7 and LLVM 15.0

Note: llir/llvm v0.3.7 will not be released until after
LLVM 15.0 has been released. The llvm15 branch currently
supports the LLVM 15.0 rc1 release.

Author: mewmew

README.md

@@ -22,6 +22,7 @@ go get -u github.com/llir/llvm/...
 
 Map between `llir/llvm` tagged releases and LLVM release versions.
 
+* [llir/llvm v0.3.7](https://github.com/llir/llvm/tree/v0.3.7): LLVM 15.0
 * [llir/llvm v0.3.6](https://github.com/llir/llvm/tree/v0.3.6): LLVM 14.0
 * [llir/llvm v0.3.5](https://github.com/llir/llvm/tree/v0.3.5): LLVM 13.0
 * [llir/llvm v0.3.4](https://github.com/llir/llvm/tree/v0.3.4): LLVM 12.0