Update changelog to indicate that 1.8 release is in progress. (#3918)

Signed-off-by: Peter Štibraný <[email protected]>

Author: pstibrany

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## master / unreleased
 
+## 1.8.0 in progress
+
 * [CHANGE] Alertmanager: Don't expose cluster information to tenants via the `/alertmanager/api/v1/status` API endpoint when operating with clustering enabled.
 * [CHANGE] Ingester: don't update internal "last updated" timestamp of TSDB if tenant only sends invalid samples. This affects how "idle" time is computed. #3727
 * [CHANGE] Require explicit flag `-<prefix>.tls-enabled` to enable TLS in GRPC clients. Previously it was enough to specify a TLS flag to enable TLS validation. #3156