Update credits.

CREDITS

@@ -17,4 +17,20 @@ epubgrep makes direct use of the following libraries and programs:
 
   Boost
     From: Boost organization and community
+          https://www.boost.org/
     License: BSL-1.0
+
+  gettext
+    From: GNU and community
+          https://www.gnu.org/software/gettext/
+    License: GPL-3.0-or-later, LGPL-2.1-or-later
+
+  libarchive
+    From: libarchive community
+          https://www.libarchive.org/
+    License: BSD-3-Clause, BSD-2-Clause, BSD-4-Clause, public-domain
+
+  fmt
+    From: fmt community
+          https://github.com/fmtlib/fmt
+    License: MIT