README: document pkg-config for luajit

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

README

@@ -54,7 +54,7 @@ Dependencies
 * libzip
 * libcrypto (OpenSSL)
 * libssl (OpenSSL)
-* optional: luajit or lua
+* optional: luajit or lua, most reliably used when pkg-config is available
 
 Apache configuration
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