CHANGELOG: prepare for 0.5.4 release

Author: kevinburke

CHANGELOG.md

@@ -1,3 +1,18 @@
+# v0.5.4
+
+Released on June 13, 2019. We are [looking for
+sponsors!](https://github.com/golang/dep/issues/2165)
+
+- Fix an error in the TOML example for the Gopkg.toml documentation.
+  ([#2174][2174])
+
+- Fix error when cleaning up git submodules with newer versions of git. Thanks
+  @geearu for the fix. ([#2168][2168], [#2176][2176])
+
+[2168]: https://github.com/golang/dep/pull/2168
+[2174]: https://github.com/golang/dep/pull/2174
+[2176]: https://github.com/golang/dep/pull/2176
+
 # v0.5.3
 
 Released on May 13, 2019