enr-entries/les.md: add les entry specification (#175)

Author: fjl

enr-entries/les.md

@@ -0,0 +1,32 @@
+# The "les" ENR entry
+
+This specification defines the "les" ENR entry, which provides information about the [les
+capability] provided by a node. The presence of this entry in a node's ENR indicates that
+the node is acting as a light client server. ENRs containing the "les" entry must also
+contain an [eth entry], which provides information about the specific Ethereum blockchain
+served by LES.
+
+## Entry Format
+
+    entry-key   = "les"
+    entry-value = [ vflux-version ]
+
+At this time, the "les" entry is a single element list containing the version number of
+the 'vflux' payment protocol.
+
+In order to be compatible with future versions of this specifications, implementations
+should ignore any additional list elements in `entry-value`.
+
+## Change Log
+
+### vflux-version (March 2021)
+
+In March 2021, the les entry was updated to include the vflux version number.
+
+### Initial Version (October 2019)
+
+The initial version of the les entry was an empty list with the sole purpose of
+signaling LES server support.
+
+[les capability]: ../caps/les.md
+[eth entry]: ./eth.md