[ffigen] add LLVM installation instructions for Fedora (#584)

pkgs/ffigen/README.md

diff --git a/pkgs/ffigen/README.md b/pkgs/ffigen/README.md
index b703655..38c66e5 100644
--- a/pkgs/ffigen/README.md
+++ b/pkgs/ffigen/README.md
@@ -63,8 +63,12 @@
 ## Installing LLVM
 `package:ffigen` uses LLVM. Install LLVM (9+) in the following way.
 
-#### ubuntu/linux
-1. Install libclangdev - `sudo apt-get install libclang-dev`.
+#### Linux
+1. Install libclangdev.
+
+   With apt-get: `sudo apt-get install libclang-dev`.
+
+   With dnf: `sudo dnf install clang-devel`.
 
 #### Windows
 1. Install Visual Studio with C++ development support.