README: add delete blob contract note

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Iac2768eebcf1ab28e28cdba601574dbede4fc9ec
diff --git a/README.md b/README.md
index e3966c7..945289b 100644
--- a/README.md
+++ b/README.md
@@ -380,3 +380,5 @@
     if that session is already open (e.g. `stat()` or `commit()`).
     *   The caveat is the open command where the session is provided to the
         handler within the call.
+*   The blob manager will only call `delete()` on a blob if there are no open
+    sessions to that blob id.