Rename index.md to README.md to make it automatically show up

Also add a heading to each file.

No-Try: True
Bug: webrtc:11335
Change-Id: I5e935741662558e72e417fa80a48c5ecda66c5f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169854
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30702}

docs/native-code/rtp-hdrext/abs-send-time/README.md

@@ -0,0 +1,31 @@
+# Absolute Send Time
+
+The Absolute Send Time extension is used to stamp RTP packets with a timestamp
+showing the departure time from the system that put this packet on the wire
+(or as close to this as we can manage). Contact <solenberg@google.com> for
+more info.
+
+Name: "Absolute Sender Time" ; "RTP Header Extension for Absolute Sender Time"
+
+Formal name: <http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time>
+
+SDP "a= name": "abs-send-time" ; this is also used in client/cloud signaling.
+
+Not unlike [RTP with TFRC](http://tools.ietf.org/html/draft-ietf-avt-tfrc-profile-10#section-5)
+
+Wire format: 1-byte extension, 3 bytes of data. total 4 bytes extra per packet
+(plus shared 4 bytes for all extensions present: 2 byte magic word 0xBEDE, 2
+byte # of extensions). Will in practice replace the "toffset" extension so we
+should see no long term increase in traffic as a result.
+
+Encoding: Timestamp is in seconds, 24 bit 6.18 fixed point, yielding 64s
+wraparound and 3.8us resolution (one increment for each 477 bytes going out on
+a 1Gbps interface).
+
+Relation to NTP timestamps: abs_send_time_24 = (ntp_timestamp_64 >> 14) &
+0x00ffffff ; NTP timestamp is 32 bits for whole seconds, 32 bits fraction of
+second.
+
+Notes: Packets are time stamped when going out, preferably close to metal.
+Intermediate RTP relays (entities possibly altering the stream) should remove
+the extension or set its own timestamp.