Update 06-params.md

Expanded the introduction a  little to make it clearer as to why we would want to reference input parameters and how.

Author: longr

_episodes/06-params.md

@@ -12,9 +12,12 @@ keypoints:
 ---
 In a previous example, we extracted a file using the "tar" program.
 However, that example was very limited because it assumed that the file
-we were interested in was called "hello.txt".  In this example, you will
-see how to reference the value of input parameters dynamically from other
-fields.
+we were interested in was called "hello.txt", and this was written into the
+`.cwl` file. This is not the best way to do this, as the filename may vary or
+be dependant on the input file(s) used.  To avoid this we can specify the 
+name of the file we want in the job parameters file (`.yml`). In this example,
+you will see how to reference the value of input parameters dynamically 
+from other fields, which will allow us to then specify the name of the file to extract.
 
 *tar-param.cwl*