added runtime requirement episode

Author: Toby Hodges

_episodes/14-runtime.md

@@ -0,0 +1,40 @@
+---
+title: "Creating Files at Runtime"
+teaching: 10
+exercises: 0
+questions:
+- "What do I do when I want to create values dynamically and CWL doesn't
+provide a built-in way of doing so?"
+objectives:
+- "Learn how to insert JavaScript expressions into a CWL description."
+keypoints:
+- "Use `InitialWorkDirRequirement` to specify input files that need to be
+created during tool runtime."
+---
+Sometimes you need to create a file on the fly from input parameters,
+such as tools which expect to read their input configuration from a file
+rather than the command line parameters.  To do this, use
+`InitialWorkDirRequirement`.
+
+*createfile.cwl*
+
+```
+{% include cwl/createfile.cwl %}
+```
+
+*echo-job.yml*
+
+```
+{% include cwl/echo-job.yml %}
+```
+
+Now invoke `cwl-runner` with the tool wrapper and the input object on the
+command line:
+
+```
+$ cwltool createfile.cwl echo-job.yml
+[job 140528604979344] /home/example$ cat example.conf
+CONFIGVAR=Hello world!
+Final process status is success
+{}
+```