README.md: Clarify the precise `unified filesystem' view needs

Reported-by: Mark Wooding <mdw@distorted.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/README.md b/README.md
index e6b389c8f371a18c5835f1fee776353da4e24734..d730874503ca1305fbd80e13a3fe391deebb5dfa 100644 (file)
--- a/README.md
+++ b/README.md
@@ -308,7 +308,15 @@ Limitations and bugs
     left by an interrupted run.
 
   * Out of tree builds require a unified filesystem view: eg, different
-    users on the same host, NFS, or something.  This could be improved.
+    users on the same host, NFS, or something.
+
+    Specifically, the invocation and build execution environments must
+    both have visibility of the source and build directories, at the
+    same absolute pathnames.  The invocation environment must be able
+    to write to the build environment (but vice versa is not
+    required).
+
+    This could be improved.
 
   * The alternative `Cargo.lock` filename must currently be a leafname.  I
     think fixing this just involves review to check other values work