Update documentation

docs/docs/basic-usage.md

@@ -144,3 +144,19 @@ and update the lock file with the new versions.
 
     Poetry will display a Warning when executing an install command if `pyproject.lock` and `pyproject.toml`
     are not synchronized.
+
+
+## Poetry and virtualenvs
+
+When you execute the `install` command (or any other "install" commands like `add` or `remove`),
+Poetry will check if it's currently inside a virtualenv and, if not, will use an existing one
+or create a brand new one for you to always work isolated from your global Python installation.
+
+!!!warning
+
+    Since Poetry is 3.6+ only, it will create virtualenvs
+    using the standard library `venv` module.
+    
+    So if you want to work with a previous Python version in your
+    project, you will need to create your own virtualenvs using, for instance,
+    [pew](https://github.com/berdario/pew).

docs/docs/cli.md

@@ -243,6 +243,25 @@ If you do not specify the password you will be prompted to write it.
 * `--unset`: Remove the configuration element named by `setting-key`.
 * `--list`: Show the list of current config variables.
 
+## run
+
+The `run` command executes the given command inside the project's virtualenv.
+
+```bash
+poetry run python -V
+```
+
+Note that this command has no option.
+
+## check
+
+The `check` command validate the structure of the `pyproject.toml` file
+and returns a detailed report is there are any errors.
+
+```bash
+poetry check
+```
+
 ## search
 
 This command searches for packages on a remote index.