Update README with instructions for installing from dev channel

Author: bjodah

.gitignore

@@ -29,7 +29,7 @@ build*
 MANIFEST
 *.egg-info
 dist/
-.cache/
+.*cache/
 symengine.egg-info/
 
 # Temp files

README.md

@@ -23,6 +23,10 @@ See License section for information about wheels
 
     conda install python-symengine -c symengine -c conda-forge
 
+optionally, you may choose to install an early [developer preview](https://github.com/symengine/python-symengine-feedstock):
+
+    conda install python-symengine -c symengine/dev -c conda-forge
+
 ### Build from source
 
 Install prerequisites.