Re-add conda instructions (#1196)

Now that @croth1 has updated the conda-forge recipe to version 1.0.20190906054215, the text about conda installation can be returned to the README.

Author: pvanheus

README.rst

@@ -96,9 +96,15 @@ Your operating system may offer cwltool directly. For `Debian <https://tracker.d
 
   apt-get install cwltool
 
-Otherwise, to
-avoid conflicting versions of the same library,
-it is recommended to do the following:
+For MacOS X, other UNIXes or Windows packages prepared by the conda-forge project. Please follow instructions of conda-forge (https://conda-forge.org/#about) for its installation, then perform: 
+
+.. code:: bash
+
+   conda install -c conda-forge cwltool
+   
+Under the hood, conda setups virtual environments before installing `cwltool` to
+avoid conflicting versions of the same library. When installing cwltool directly,
+it is recommended to do the same manually:
 
 .. code:: bash