release: Update v0.1.0 release date and add note (#281)

Adds a note to the main readme about pre-v1 changes.

Signed-off-by: Marc Lopez <[email protected]>

Author: marclop

CHANGELOG.md

@@ -1,4 +1,4 @@
-# 0.1.0 (Unreleased)
+# 0.1.0 (March 31, 2021)
 
 BREAKING CHANGES:
 

README.md

@@ -9,6 +9,8 @@ Terraform provider for the Elastic Cloud API, including:
 * Elastic Cloud Enterprise (ECE).
 * Elasticsearch Service Private (ESSP).
 
+_Model changes might be introduced between minors until version 1.0.0 is released. Such changes and the expected impact will be detailed in the change log and the individual release notes._
+
 ## Example usage
 
 ```hcl