Add v0.15 release post to changelog
- Projects without rebar3 can be generated using the `gleam-lib` template.
## v0.15.0 - 2021-05-06
+
+[Release Blog Post](https://gleam.run/news/gleam-v0.15-released/)
+
## v0.15.0-rc1 - 2021-05-05
- Syntax highlighting of Gleam code in generated HTML documentation has been
