Enhance documentation about options for `pyenv virtualenv` (#425)

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

README.md

@@ -99,6 +99,9 @@ $ pyenv virtualenv 2.7.10 my-virtual-env-2.7.10
 will create a virtualenv based on Python 2.7.10 under `$(pyenv root)/versions` in a
 folder called `my-virtual-env-2.7.10`.
 
+`pyenv virtualenv` forwards any options to the underlying command that actually
+creates the virtual environment (`conda`, `virtualenv`, or `python -m venv`).
+See the output of `pyenv virtualenv --help` for details.
 
 ### Create virtualenv from current version
 

bin/pyenv-virtualenv

@@ -6,7 +6,11 @@
 #        pyenv virtualenv --version
 #        pyenv virtualenv --help
 #
-#   -f/--force       Install even if the version appears to be installed already
+#   -f/--force       Install even if the version appears to be installed already. Skip
+#                    prompting for confirmation
+#
+# Notable VIRTUALENV_OPTIONS passed to venv-creating executable, if applicable:
+#   -u/--upgrade     Imply --force
 #
 
 PYENV_VIRTUALENV_VERSION="1.1.5"