Proposed congestion control requirement to recharter (#241)

* Proposed congestion control requirement to recharter
* Tweak text based on Kazuho and Willy's feedback

Author: LPardue

charter/charter.md

@@ -38,7 +38,9 @@ It is chartered to pursue work in the areas detailed below:
    can be adapted to work over a reliable and bidirectional byte stream
    substrate. When the substrate is insecure, TLS will be the default
    security provider; no effort will be made to enable unprotected
-   communication without a security provider.
+   communication without a security provider. Substrates must provide
+   congestion-management capabilities applicable to their deployment
+   environments.
 
 Specifications describing how new or existing application protocols
 use the QUIC transport layer, called application protocol mappings