Add "definition version" date (#19)

README.md

@@ -290,6 +290,8 @@ _Reminder: the proposed requirements here are based on what Google currently
 uses and has found useful. We expect these requirements—and perhaps even the
 concept of levels—to evolve based on community feedback._
 
+**Definition Version: 2021-04-21**
+
 There are three SLSA levels. SLSA 3 is the current highest level and represents
 the ideal end state. SLSA 1 and 2 offer lower security guarantees but are easier
 to meet. In our experience, achieving SLSA 3 can take many years and significant
@@ -320,6 +322,8 @@ effort, so intermediate milestones are important.
 
 Each SLSA level has a set of requirements.
 
+<!-- REMINDER: Update the version date whenever changing requirements. -->
+
 <table>
  <thead>
   <tr><th colspan="2">                       <th colspan="3">Required at   </tr>