Add external module bd.

.gitmodules

@@ -16,3 +16,6 @@
 [submodule "modules/prompt/functions/pure"]
 	path = modules/prompt/external/pure
 	url = https://github.com/sindresorhus/pure.git
+[submodule "modules/bd/external"]
+	path = modules/bd/external
+	url = https://github.com/Tarrasch/zsh-bd.git

modules/bd/Readme.md

@@ -0,0 +1,18 @@
+# bd
+Jump back to a specific directory, without doing `cd ../../..`
+
+This is simply [bd](https://github.com/Tarrasch/zsh-bd) ported to a prezto module.
+
+For further information, e.g. a usage animation, I refer to the [original repository](https://github.com/Tarrasch/zsh-bd).
+
+## Usage
+```zsh
+$ mkdir -p a/b/c/d
+$ cd a/b/c/d
+$ bd b
+$ ls
+c
+```
+
+## Acknowledgements
+All credits go to [@Tarrasch](https://github.com/Tarrasch) for the zsh implementation of bd!

modules/bd/external

@@ -0,0 +1 @@
+Subproject commit 73670e1f23dd7e094cfd3083425ee2cd6b938e07

modules/bd/init.zsh

@@ -0,0 +1,2 @@
+# Source module files.
+source "${0:h}/external/bd.zsh" || return 1