Add URL data locators to launch sub-command (#920)

* initial commit of URL support for launch

* lint

* modify tests to use new data locator

* add locator unit tests

* fix typo in faq

* more lint

* update faq per PR review

docs/faq.md

@@ -94,3 +94,19 @@ This may happen, especially as we work out bugs in our installation process! Ple
 #### I'm following the developer instructions and get an error about "missing files and directories” when trying to build the client
 
 This is likely because you do not have node and npm installed, we recommend using [nvm](https://github.com/creationix/nvm) if you're new to using these tools.
+
+# Data access
+
+#### Can I use a _s3:_ or _gs:_ URL with `cellxgene launch`?
+
+Yes. Support for S3 and GCS is not enabled by default. If you wish to directly access S3 or GFS, install one or both of the following packages using `pip`:
+
+- [s3fs](https://s3fs.readthedocs.io/en/latest/) for S3 support
+- [gcsfs](https://gcsfs.readthedocs.io/en/latest/) for GCS support
+
+For example:
+
+```
+pip install s3fs
+cellxgene launch s3://mybucket.s3-us-west-2.amazonaws.com/mydata.h5ad
+```