submodules: Add maddy markdown parser (header-only library)

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "submodules/qssb.h"]
 	path = submodules/qssb.h
 	url = https://gitea.quitesimple.org/crtxcr/qssb.h.git
+[submodule "submodules/maddy"]
+	path = submodules/maddy
+	url = https://github.com/progsource/maddy