Merge pull request #684 from ethereum/vbuterin-patch-5

Developer notice

specs/core/1_shard-data-chains.md

@@ -2,6 +2,8 @@
 
 **NOTICE**: This document is a work-in-progress for researchers and implementers. It reflects recent spec changes and takes precedence over the [Python proof-of-concept implementation](https://github.com/ethereum/beacon_chain).
 
+At the current stage, Phase 1, while fundamentally feature-complete, is still subject to change. Development teams with spare resources may consider starting on the "Shard chains and crosslink data" section; at least basic properties, such as the fact that a shard block can get created every slot and is dependent on both a parent block in the same shard and a beacon chain block at or before that same slot, are unlikely to change, though details are likely to undergo similar kinds of changes to what Phase 0 has undergone since the start of the year.
+
 ## Table of contents
 
 <!-- TOC -->
@@ -15,6 +17,7 @@
             - [Time parameters](#time-parameters)
             - [Max operations per block](#max-operations-per-block)
             - [Signature domains](#signature-domains)
+    - [Shard chains and crosslink data](#shard-chains-and-crosslink-data)
       - [Helper functions](#helper-functions)
             - [`get_split_offset`](#get_split_offset)
             - [`get_shuffled_committee`](#get_shuffled_committee)
@@ -98,6 +101,8 @@ Phase 1 depends upon all of the constants defined in [Phase 0](0_beacon-chain.md
 | `DOMAIN_SHARD_ATTESTER`| 130             |
 | `DOMAIN_CUSTODY_SUBKEY`| 131             |
 
+# Shard chains and crosslink data
+
 ## Helper functions
 
 #### `get_split_offset`