added notes about DLT allocation

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+DLT and LINKTYPE allocation
+===========================
+
+DLT_ types live in pcap/dlt.h.  They can be requested by the community on a
+First-Come First-Served basis [i.e. https://tools.ietf.org/html/rfc8126#section-4.4 ]
+(Although libpcap is not at this time an IETF specification, there have been
+some as yet-incomplete efforts to do this).
+
+The Tcpdump Group prefers to link to an open specification on the new DLT_
+type,  but they are available for closed, proprietary projects as well.
+In that case, a stable email address suffices so that someone who finds
+an unknown DLT_ type can investigate.
+We prefer to give out unambiguous numbers, and we try to do it as quickly
+as possible, but DLT_USERx is available while you wait.
+
+Note that DLT_ types are, in theory, private to the capture mechanism and can
+in some cases be operating system specific, and so a second set of values,
+LINKTYPE_ is allocated for actually writing to pcap files.  As much as
+possible going forward, the DLT_ and LINKTYPE_ value are identical, however,
+this was not always the case.  See pcap-common.c.
+
+The LINKTYPE_ values are not exported, but are on pcap-common.c only.
+
+DEVELOPER NOTES
+---------------
+
+When allocating a new DLT_ value, a corresponding value needs to be
+added to pcap-common.c.
+It not necessary to copy to commonts from dlt.h to pcap-common.c.
+
+
+
+
+
+