docs: updated $exp_date to epoch timestamp

Author: ethanneff

README.md

@@ -401,7 +401,7 @@
       | $publicly_indexable | `1` | Cannot modify here. Needs to be set by the Branch Universal Object
       | $keywords | | Keywords for which this content should be discovered by. Just assign an array of strings with the keywords you’d like to use
       | $canonical_identifier | | This is the unique identifier for content that will help Branch dedupe across many instances of the same thing. Suitable options: a website with pathing, or a database with identifiers for entities
-      | $exp_date | `0` | Cannot modify here. Needs to be set by the Branch Universal Object
+      | $exp_date | `0` | Cannot modify here. Needs to be set by the Branch Universal Object. Must be epoch timestamp with milliseconds
       | $content_type | | This is a label for the type of content present. Apple recommends that you use uniform type identifier as described here
 
     - DeepView