add forward slash path requirement to common principles

src/02-common-principles.md

@@ -650,6 +650,16 @@ it refers to. For example `sub-01` refers to the `sub` entity (a
 subject) with the label `01`. The `sub-` prefix is not part of the subject
 label, but must be included in file names (similarly to other key names).
 
+## Specification of paths
+
+Several metadata fields in BIDS require the specification of paths, that is
+a string of characters used to uniquely identify a location in a directory
+structure. For example the `IntendedFor` or `AssociatedEmptyroom` metadata fields.
+
+Throughout BIDS all such paths MUST be specified using the slash character (`/`),
+regardless of the operating system that a particular dataset is curated on
+or used on.
+
 ## Uniform Resource Indicator
 
 A Uniform Resource Indicator (URI) is a string referring to a resource and SHOULD