Update README.md

README.md

@@ -22,6 +22,13 @@ references, and associated materials.
    be named with the bibtex key as the base of the file name, and the
    appropriate extension from the table below according to document purpose.
 
+## Copyright Notes
+You should check with the publisher's T&Cs (in particular the Copyright waiver
+form) to determine which version of the paper you are allowed to publish here.
+Often it is only the pre-print edition that is passed to the publisher
+**before** they make any changes. The final proof copy and the publisher's own
+copy is often not permitted to be published.
+
 ## BibTeX Record Structure
 Record keys have the preferred format of `<first author surname><year><first
 word of title>`
@@ -29,9 +36,13 @@ word of title>`
 Fields:
 * **doi**: Please remove the `http://dx.doi.org/` prefix, so the doi is of the
   form `nn.nnnn/....`]
+* **url**: This field should **not** contain a duplicate of the doi. It
+  generally only used to link to the publisher's version if the publisher does
+  not also issue a DOI.
 * **file**: This field is added automatically by the JabRef reference manager,
-  it is ignored by the website. See next section for file matching on the
-  website.
+  it is ignored by the website. It is mostly useful as a quick check in the
+  JabRef user interface that all publications have an associated file. See next
+  section for file matching on the website.
 
 ## Publication File Extension Types
 Publication file types are listed in the configuration file
@@ -61,10 +72,3 @@ as a checkout before build/deploy. Post-checkin/test pass hooks can be
 configured on this repository to retrigger a website submodule update/rebuild as
 desired.
 
-
-## Copyright Notes
-You should check with the publisher's T&Cs (in particular the Copyright waiver
-form) to determine which version of the paper you are allowed to publish here.
-Often it is only the pre-print edition that is passed to the publisher
-**before** they make any changes. The final proof copy and the publisher's own
-copy is often not permitted to be published.