Extension-indexers: add placeholder spec (#9861)

Author: jcouv

proposals/extension-indexers.md

@@ -0,0 +1,21 @@
+# Extension indexers
+
+[!INCLUDE[Specletdisclaimer](../speclet-disclaimer.md)]
+
+## Declaration
+
+Like all extension members, extension indexers are declared within an extension block:
+
+``` c#
+public static class Indexers
+{
+    extension(int i)
+    {
+        public bool this[int bit] => ...;
+    }
+}
+```
+
+Extension indexers declarations generally follow the rules for 
+non-extension [indexers in the Standard](https://github.com/dotnet/csharpstandard/blob/standard-v7/standard/classes.md#159-indexers).
+