Add proposal for SSU SessionRequest extended options

i2p2www/pages/site/docs/spec/ssu.html

@@ -301,13 +301,27 @@ Typical size including header, in current implementation: 304 (IPv4) or 320 (IPv
 (before non-mod-16 padding)
 </p>
 
+<h4>Extended Options</h4>
+<p>
+<i>Note: This is a proposal, to be implemented in 0.9.24. Subject to change.</i>
+</p>
+<ul><li>
+Minimum length: 3 (option length byte + 2 bytes)
+</li><li>Option length: 2 minimum
+</li><li>2 bytes flags:<pre>
+Bit order: 15...76543210 (bit 15 is MSB)
+   bit 0: 1 for Alice to request a relay tag from Bob in the SessionCreated response,
+          0 if Alice does not need a relay tag.
+          Note that "1" is the default if no extended options are present
+   bits 15-1: unused, set to 0 for compatibility with future uses
+</pre></ul>
+
+
 <h4>Notes</h4>
 <ul><li>
 IPv4 and IPv6 addresses are supported.
 </li><li>
 The uninterpreted data could possibly be used in the future for challenges.
-</li><li>
-Extended options in the header: TBD
 </li></ul>