Added documentation for lists

_docs/mod/index.md

@@ -5,4 +5,5 @@ Modules of the standard library:
 - [core](/mod/core): Core builtins
 - [csv](/mod/csv): Functions for operating over CSV data.
 - [fs](/mod/fs): File system functions
+- [lists](/mod/lists): List utilities
 - [os](/mod/os): Operating system functions 

_docs/mod/list.md

@@ -0,0 +1,13 @@
+---
+---
+
+# List Builtins
+
+### add
+
+```
+lists:add LIST [ARGS...]
+```
+
+Adds values to the end of the list. Returns the modified list. 
+