MAINT: add absolute link for CONTRIBUTING.md (caporaso-lab#80)

This is for convenience as http://mockrobiota.caporasolab.us/ is built directly from this page, so a relative link won't work there.

README.md

@@ -11,7 +11,7 @@ A public resource for microbiome bioinformatics benchmarking using artifically c
 mockrobiota distributes metadata and other materials associated with mock community datasets for public use, as well as links to raw sequencing data that are hosted externally. As raw data are hosted externally, mockrobiota cannot ensure the integrity or quality of raw data. Critical errors associated with any mockrobiota materials or raw data should be [reported here](https://github.com/caporaso-lab/mockrobiota/issues). For minor issues, e.g., formatting quirks associated with raw data, or notes on the usage of a particular dataset, document this information in the ``README.md`` file, which  is located in the [home directory of each dataset](https://github.com/caporaso-lab/mockrobiota/tree/master/data).
 
 ## Contributing
-We welcome and encourage new submissions to the mockrobiota family! :tada: If you have a mock community dataset that you would like to contribute, or wish to make changes/additions to any of the materials contained in mockrobiota, please review our [contributing guidelines](./CONTRIBUTING.md).
+We welcome and encourage new submissions to the mockrobiota family! If you have a mock community dataset that you would like to contribute, or wish to make changes/additions to any of the materials contained in mockrobiota, please review our [contributing guidelines](https://github.com/caporaso-lab/mockrobiota/blob/master/CONTRIBUTING.md).
 
 ## Citing mockrobiota
 If you use any materials contained in mockrobiota, please cite [this article](http://msystems.asm.org/content/1/5/e00062-16):