Add a link to Firecracker

The Firecracker virtual machine monitor was originally built to use
musl, though now it can also be built with glibc. The binary releases
remain built with musl.

projects-using-musl.md

@@ -110,6 +110,9 @@ Third-party projects using or building on musl:
 - [NodeOS] linux with Node.js as userspace
 - [Rust] is a programming language with musl supported as a cross-compilation
   option
+- [Firecracker VMM] is an open source virtualization technology that
+  is purpose-built for creating and managing secure, multi-tenant
+  container and function-based services.
 
 [simplecct]: https://code.google.com/p/simplecct/
 [ELLCC]: http://ellcc.org/
@@ -123,4 +126,4 @@ Third-party projects using or building on musl:
 [seL4]: https://github.com/seL4/libmuslc
 [NodeOS]: https://github.com/NodeOS/NodeOS
 [Rust]: http://www.rust-lang.org/
-
+[Firecracker VMM]: https://firecracker-microvm.github.io/