README: document pkg-config for luajit

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
diff --git a/README b/README
index 7b72842..9f26959 100644
--- a/README
+++ b/README
@@ -54,7 +54,7 @@
 * libzip
 * libcrypto (OpenSSL)
 * libssl (OpenSSL)
-* optional: luajit or lua
+* optional: luajit or lua, most reliably used when pkg-config is available
 
 Apache configuration
 --------------------