readme changes to reflect new versioning schema, again.

README.md

@@ -6,7 +6,7 @@ For all enthusiasts who just want to have a running server:
 - or with version: `docker pull qwick/terasology:1.2.1-alpha`
 
 For all developers who want a nightly build for debugging or just testing purposes:
-- `docker pull qwick/terasology:nightly-latest`
+- `docker pull qwick/terasology:unstable-latest`
 - a versioned tag is available too.
 
 Versioned tags may change over time.
@@ -18,8 +18,8 @@ To avoid confusion here the docker image tag schema:
 Stable:
 - `<engineVersion>-<displayVersion>` e.g. `1.2.1-alpha`
 
-Nightly:
-- `nightly-<engineVersion>-<buildNumber>` e.g. `nightly-1.2.2-SNAPSHOT-1806`
+Unstable:
+- `unstable-<engineVersion>-<buildNumber>` e.g. `unstable-1.2.2-SNAPSHOT-1806`
 
 
 ## Small guide