add conda instructions for rc install

Author: alimanfoo

docs/release.rst

@@ -7,10 +7,14 @@ Release notes
 -------------------------
 
 Version 2.2.0 is currently at the release candidate stage. To install the latest release
-candidate version::
+candidate version using pip::
 
     $ pip install --pre zarr
 
+Alternatively, to install the latest release candidate using conda::
+
+    $ conda install -c conda-forge/label/rc zarr
+
 Enhancements
 ~~~~~~~~~~~~