Quick add of boundary option and update to readme

README.md

@@ -227,6 +227,16 @@ This command remeshes the atoms/elements within the group to the new element siz
 
 If this option is passed then all files are automatically overwritten without asking the user.
 
+### Option boundary
+
+``` 
+-boundary box_bc
+```
+
+This allows the user to specify the boundary conditions for the model being outputted. The format is a 3 character string with `p` indicating periodic and `s` indicating shrink-wrapped.
+
+**Example:** `-boundary psp`
+
 ## Position Specification
 
 Specifying positions in cacmb can be done through a variety of ways. Examples of each format is shown below.