Give guidance on interoperable field names

Fixes #30

draft-ietf-httpbis-semantics-latest.xml

@@ -1273,6 +1273,13 @@ Content-Type: text/plain
    a Description field, it might use "Foo-Desc"; "Description" is too generic,
    and "Foo-Description" is needlessly long.
 </t>
+<t>
+   While the field-name syntax is defined to allow any token character, in
+   practice some implementations place limits on the characters they accept
+   in field-names. To be interoperable, new field names &SHOULD; constrain
+   themselves to alphanumeric characters, "-", "_", "." and ".", and &SHOULD;
+   begin with an alphanumeric character.
+</t>
 <t>
    Field names ought not be prefixed with "X-"; see
    <xref target="BCP178"/> for further information.
@@ -12030,6 +12037,7 @@ Content-Encoding: gzip
   <li>In <xref target="header.fields"/>, introduce field terminology and distinguish between field line values and field values; use terminology consistently throughout (<eref target="https://github.com/httpwg/http-core/issues/111"/>)</li>
   <li>Moved #rule definition into <xref target="field.values"/> and whitespace into <xref target="notation"/> (<eref target="https://github.com/httpwg/http-core/issues/162"/>)</li>
   <li>In <xref target="http.userinfo"/>, be more correct about what was deprecated by RFC 3986 (<eref target="https://github.com/httpwg/http-core/issues/278"/>, <eref target="https://www.rfc-editor.org/errata/eid5964"/>)</li>
+  <li>In <xref target="field.names"/>, give guidance on interoperable field names (<eref target="https://github.com/httpwg/http-core/issues/30"/>)</li>
 </ul>
 </section>
 </section>