DOCS: Fix error in usage documentation

In the documentation on using the feature classes directly, the line to enable all features in the first order feature class was missing.
This resulted in no features being calculated, as on the feature class level, all features are disabled by default.

CHANGES.rst

@@ -59,6 +59,7 @@ Documentation
   (`#291 <https://github.com/Radiomics/pyradiomics/pull/291>`_)
 - Include contributing guidelines in sphinx-generated documentation and add a section on sharing parameter files.
   (`#294 <https://github.com/Radiomics/pyradiomics/pull/294>`_)
+- Insert missing line to enable all features in documentation on using the feature classes directly.
 
 Examples
 ########

docs/usage.rst

@@ -156,6 +156,7 @@ Using feature classes directly
 * Calculate the first order features::
 
      firstOrderFeatures = firstorder.RadiomicsFirstOrder(image,mask)
+     firstOrderFeatures.enableAllFeatures()  # On the feature class level, all features are disabled by default.
      firstOrderFeatures.calculateFeatures()
      for (key,val) in six.iteritems(firstOrderFeatures.featureValues):
        print("\t%s: %s" % (key, val))