Update RELEASES.md info for 0.15.1

RELEASES.md

@@ -4,7 +4,7 @@ Go [here](https://github.com/open-source-economics/Tax-Calculator/pulls?q=is%3Ap
 for a complete commit history.
 
 
-2018-01-?? Release 0.15.1
+2018-01-?? Release 0.15.2
 -------------------------
 (last merged pull request is
 [#18xx](https://github.com/open-source-economics/Tax-Calculator/pull/18xx))
@@ -21,6 +21,23 @@ for a complete commit history.
 - None
 
 
+2018-01-20 Release 0.15.1
+-------------------------
+(last merged pull request is
+[#1838](https://github.com/open-source-economics/Tax-Calculator/pull/1838))
+
+**API Changes**
+- None
+
+**New Features**
+- Add `cpi_inflatable` field for each parameter in the four JSON parameter files
+  [[#1838](https://github.com/open-source-economics/Tax-Calculator/pull/1838)
+  by Martin Holmer as requested by Hank Doupe]
+
+**Bug Fixes**
+- None
+
+
 2018-01-18 Release 0.15.0
 -------------------------
 (last merged pull request is