[MISC] add "forward slash" requirement for paths to common principles (bids-standard#867)

* add forward slash path requirement to common principles

* clarify absolute paths and relative paths

* go from must to should

Author: sappelhoff

src/02-common-principles.md

@@ -661,6 +661,20 @@ 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 filenames (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.
+
+Paths SHOULD NOT be absolute local paths,
+because these might break when a dataset is used on a different machine.
+It is RECOMMENDED that all paths specified in a BIDS dataset are relative paths,
+as specified in the respective descriptions of metadata fields that require the use of paths.
+
 ## Uniform Resource Indicator
 
 A Uniform Resource Indicator (URI) is a string referring to a resource and SHOULD