Update README.md

Author: jfy133

README.md

@@ -7,7 +7,7 @@ Test data to be used for automated testing with the nf-core pipelines
 
 nf-core is a collection of high quality Nextflow pipelines. This repository contains various files for CI and unit testing of nf-core pipelines and infrastructure.
 
-The principle for nf-core test data is as small as possible, as large as necessary. Always ask for guidance on the [nf-core slack](https://nf-co.re/join) before adding new test data.
+The principle for nf-core test data is as small as possible, as large as necessary. Please see the [guidelines](https://nf-co.re/docs/contributing/test_data_guidelines) for more detailed information. Always ask for guidance on the [nf-core slack](https://nf-co.re/join) before adding new test data.
 
 ## Documentation