Add warning to NVStore.md

Add notice of deprecation.

Author: Amanda Butler

docs/api/storage/NVStore.md

@@ -1,5 +1,7 @@
 # NVStore
 
+<span class="warnings">**Warning:** This API will be deprecated. In its place, please use [KVStore](../apis/kvstore.html).</span>
+
 <span class="images">![](https://os.mbed.com/docs/development/mbed-os-api-doxy/class_n_v_store.png)<span>NVStore class hierarchy</span></span>
 
 NVStore is a lightweight module that stores data by keys in the internal flash for security purposes. For each item key, the NVStore module provides the ability to set the item data or get it. Newly added values are added to the end of the existing data, superseding the previous value that was there for the same key. The NVStore module ensures that power failures don't harm existing data during any operation.