add library chapter tutorial13

booksrc/tutorial13.md

@@ -0,0 +1,18 @@
+# Writing a library
+
+I would advise to only expose an `mycrate::ErrorKind` and type alias `mycrate::Error` to `ChainError<mycrate::ErrorKind>`
+so you can tell your library users to use the `.kind()` method as `std::io::Error` does.
+
+If you later decide to make your own `Error` implementation, your library users don't
+have to change much or anything.
+
+~~~rust
+# #[allow(dead_code)]
+# #[macro_use]
+# pub mod chainerror {
+{{#includecomment ../src/lib.rs}}
+# }
+pub mod mycrate {
+    use crate::chainerror::*; // omit the `crate::` part
+{{#include ../examples/tutorial13.rs:3:}}
+~~~