Added validation section to README.md, error logic if womtool and min…

…iwdl are disabled

README.md

@@ -113,6 +113,13 @@ Cromshell is a CLI for submitting workflows to a Cromwell server and monitoring/
      * `-c/--color` Color outliers in task level cost results.
      * `-d/--detailed` Get the cost for a workflow at the task level.
 
+   #### Validate WDL
+   * `validate <wdl> --dependencies-zip [wdl_zip_file]`
+     * Validate a WDL file.
+     * Runs both miniwdl and womtool validation by default, but can be configured to run only one or the other.
+     * Womtool validation via Cromwell server API does not support validation of imported files, however miniwdl does.
+     * `--dependencies-zip`  MiniWDL option: ZIP file or directory containing workflow source files that are used to resolve local imports.
+
 ## Features:
  * Running `submit` will create a new folder in the `~/.cromshell/${CROMWELL_URL}/` directory named with the cromwell job id of the newly submitted job.  
  It will copy your wdl and json inputs into the folder for reproducibility.  

src/cromshell/validate/command.py

@@ -80,6 +80,10 @@ def main(
 
     return_code = 0
 
+    if no_womtool and no_miniwdl:
+        LOGGER.error("At least one validation tool must be enabled.")
+        raise MissingArgumentError("At least one validation tool must be enabled.")
+
     if not no_womtool:
         if not wdl_json:
             LOGGER.error("WDL JSON file is required.")