Add comment to the norwfs version of SetRef

Signed-off-by: Javi Fontan <jfontan@gmail.com>

Author: jfontan

storage/filesystem/internal/dotgit/dotgit_setref_norwfs.go

@@ -4,6 +4,13 @@ package dotgit
 
 import "gopkg.in/src-d/go-git.v4/plumbing"
 
+// There are some filesystems tha don't support opening files in RDWD mode.
+// In these filesystems the standard SetRef function can not be used as i
+// reads the reference file to check that it's not modified before updating it.
+//
+// This version of the function writes the reference without extra checks
+// making it compatible with these simple filesystems. This is usually not
+// a problem as they should be accessed by only one process at a time.
 func (d *DotGit) setRef(fileName, content string, old *plumbing.Reference) error {
 	f, err := d.fs.Create(fileName)
 	if err != nil {