pyenv virtualenv: add page (#2522)

pages/common/pyenv-virtualenv.md

@@ -0,0 +1,11 @@
+# pyenv virtualenv
+
+> Create virtual environments based on one's installed Python distributions.
+
+- Create a new Python 3.6.6 virtual environment:
+
+`pyenv virtualenv {{3.6.6}} {{my-virtual-env-3.6.6}}`
+
+- List all existing virtual environments:
+
+`pyenv virtualenvs`