Auto merge of #2401 - nathan-at-least:doc-1.0.9-deprecation-policy, r=nathan-at-least

Define the deprecation policy for 1.0.9.

This explicitly avoids naming any release-specific data so we can leave
this text unchanged for upcoming releases.

ref: #2398

README.md

@@ -23,6 +23,14 @@ See important security warnings in
 
 **Zcash is unfinished and highly experimental.** Use at your own risk.
 
+Deprecation Policy
+------------------
+
+This release is considered deprecated 16 weeks after the release day. There
+is an automatic deprecation shutdown feature which will halt the node some
+time after this 16 week time period. The automatic feature is based on block
+height and can be explicitly disabled.
+
 Where do I begin?
 -----------------
 We have a guide for joining the main Zcash network: