Document the use of struct tags to map the dissimilar fields.

README.md

diff --git a/README.md b/README.md
index 9fa1933..e7769e9 100644
--- a/README.md
+++ b/README.md
@@ -433,6 +433,7 @@
 type config struct {
 	Port int
 	Name string
+	PathMap string `mapstructure:"path_map"`
 }
 
 var C config