Add 7.15.1 release notes

Author: sethmlarson

docs/guide/release-notes.asciidoc

@@ -1,6 +1,7 @@
 [[release-notes]]
 == Release notes
 
+* <<rn-7-15-1>>
 * <<rn-7-15-0>>
 * <<rn-7-14-0>>
 * <<rn-7-13-4>>
@@ -30,6 +31,22 @@
 * <<rn-7-0-0>>
 
 
+[discrete]
+[[rn-7-15-1]]
+=== 7.15.1 (2021-10-14)
+
+[discrete]
+==== Client
+
+- Fixed a performance regression in `JSONSerializer.default()` when `numpy` and `pandas` weren't installed.
+- Changed the `DeprecationWarning` for the `body` parameter to be a "removed in a future version" instead of "removed in 8.0" in line with the 8.0 roadmap.
+
+[discrete]
+==== Search
+
+- The `index` parameter of the `open_point_in_time` API is now required, was optional.
+
+
 [discrete]
 [[rn-7-15-0]]
 === 7.15.0 (2021-09-22)