Adds slack badge

Minor fix to workflow parser error message.

README.md

@@ -4,9 +4,9 @@
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 Popper is a tool for defining and executing [container-native][cn] 
 workflows in Docker, as well as [other container engines][engines]. 

cli/popper/parser.py

@@ -407,7 +407,7 @@ def parse_substitutions(self, substitutions=None, allow_loose=False):
                 wf_block['env'] = temp_dict
 
         if not allow_loose and len(substitution_dict) != len(used):
-            log.fail("Not all given substitutions are used in"
+            log.fail("Not all given substitutions are used in "
                      "the workflow file")
 
     @staticmethod