chore(docs) Use relative links for local MD files (#341)

This has pros and cons. On the downside it means some places that fetch
just the readme and not other files *and* don't render using the project
base URL might have broken links. On the upside all forks, branches, and
even distributed packages will link to their own respective local files
and versions.

README.md

@@ -59,7 +59,7 @@ Python standard libraries.
 
 ## License
 
-Penlight is distributed under the [MIT license](https://github.com/lunarmodules/Penlight/blob/master/LICENSE.md)
+Penlight is distributed under the [MIT license](LICENSE.md).
 
 ## Installation
 
@@ -83,13 +83,12 @@ through LuaRocks.  Then it's a simple matter of running `ldoc .` from the repo.
 
 ## Contributing
 
-Contributions are most welcome, please check the [contribution guidelines](https://github.com/lunarmodules/Penlight/blob/master/CONTRIBUTING.md).
+Contributions are most welcome, please check the [contribution guidelines](CONTRIBUTING.md).
 
 ## Running tests
 
 Execute `lua run.lua tests` to run the tests. Execute `lua run.lua examples` to run examples.
 
 ## History
 
-For a complete history of the development of Penlight, please check the
-[changelog](https://github.com/lunarmodules/Penlight/blob/master/CHANGELOG.md).
+For a complete history of the development of Penlight, please check the [changelog](CHANGELOG.md).