Clarify that the sponsor table for PGCA only shows "Friend" and above.

templates/sponsors/financial.html

@@ -53,6 +53,10 @@ levels are used:</p>
 
 <h3>PostgreSQL Community Association Sponsors</h3>
 
+<p>
+    Current sponsors at the Friend level and above include:
+</p>
+
 <table class="table table-striped">
   <thead>
     <tr>