Add README per RFC

Author: ldionne

third-party/README.md

@@ -0,0 +1,10 @@
+# Third-party LLVM dependencies
+
+This subfolder contains third-party dependencies used in various components of LLVM.
+Integrating a new third-party dependency generally requires it to be licensed under
+the Apache-with-LLVM-exception license. For integrating code under other licenses,
+please follow the process explained in the [LLVM Developer Policy](https://llvm.org/docs/DeveloperPolicy.html#copyright-license-and-patents).
+
+In particular, Boost.Math is cleared for usage within the libc++ compiled library
+exclusively as discussed in [this RFC](https://discourse.llvm.org/t/rfc-libc-taking-a-dependency-on-boost-math-for-the-c-17-math-special-functions).
+Do not use it in other parts of LLVM without prior discussion (and update to this documentation).