Define Alternate Steering Group Representative

And add one for Mozilla.

Fixes #223.

Author: zcorpan

README.md

@@ -58,7 +58,7 @@ Workstream Participant Agreement which is maintained as
 * Anne van Kesteren (Apple) [@annevk](https://github.com/annevk)
 * Chris Wilson (Google) [@cwilso](https://github.com/cwilso)
 * Diego González (Microsoft) [@diekus](https://github.com/diekus)
-* Tantek Çelik (Mozilla) [@tantek](https://github.com/tantek)
+* Tantek Çelik (Mozilla) [@tantek](https://github.com/tantek) (Alternate: Simon Pieters [@zcorpan](https://github.com/zcorpan))
 
 ### Changing a Steering Group representative
 

SG Agreement.md

@@ -43,6 +43,12 @@ its interests, cast votes, and otherwise represent the interests of the [Steerin
 [Steering Group Member] may replace its [Steering Group Representative] at any time in its sole
 discretion.
 
+Each [Steering Group Member] may optionally appoint an individual
+"<a id="alternate-steering-group-representative">**Alternate Steering Group Representative**</a>".
+The [Steering Group Representative] may delegate any of their rights and responsibilities to the
+[Alternate Steering Group Representative]. Only one of the [Steering Group Representative] or
+[Alternate Steering Group Representative] can communicate on a given subject at any one time.
+
 ### 3.2. Functions
 
 #### A. Mission
@@ -223,6 +229,7 @@ this [Agreement] on behalf of the identified [Steering Group Member].
 [Steering Group]: ./SG%20Agreement.md#steering-group
 [Steering Group Member]: ./SG%20Agreement.md#steering-group-member
 [Steering Group Representative]: ./SG%20Agreement.md#steering-group-representative
+[Alternate Steering Group Representative]: ./SG%20Agreement.md#alternate-steering-group-representative
 [Supermajority]: ./SG%20Agreement.md#supermajority
 [WHATWG Principles]: ./Principles.md