readme: add changelog for 1.2.1

mgeisler · mgeisler · commit fc5d1d5ee9ad · 2019-03-30T11:19:01.000+01:00
This seems to have been forgotten in 703e9c2.
diff --git a/README.md b/README.md
@@ -45,6 +45,13 @@ unicode-segmentation = "1.2.1"
 
 # Change Log
 
+## 1.2.1
+
+* [#37](https://github.com/unicode-rs/unicode-segmentation/pull/37):
+  Fix panic in `provide_context`.
+* [#40](https://github.com/unicode-rs/unicode-segmentation/pull/40):
+  Fix crash in `prev_boundary`.
+
 ## 1.2.0
 
 * New `GraphemeCursor` API allows random access and bidirectional iteration.
