[Merge on next release] Document how to install cellxgene prepare (#889)

* Document how to install cellxgene prepare

after pr #887 merged

* formatting

* remove reference to cellxgene[louvain]

docs/data.md

@@ -14,6 +14,14 @@ description: Data
 
 `cellxgene prepare` is not meant as a way to formally process or analyze your data. It's simply a utility for quickly wrangling your data into cellxgene-compatible format and computing a "vanilla" embedding so you can try out `cellxgene` and get a general sense of a dataset.
 
+#### How do I install `cellxgene prepare`?
+
+The `cellxgene prepare` command is an optional install that you can install alongside `cellxgene launch` by running
+
+```
+pip install cellxgene[prepare]
+```
+
 #### What input formats does it accept?
 
 Currently, we accept `h5ad` and `loom` files, as well as `10x` directories, and are hoping to accept more formats in the future.
@@ -52,9 +60,9 @@ Let's look at what `prepare` is doing to our data, and how each step relates to
 
 # Example datasets to use with cellxgene
 
-
-**To download and use these datasets, run:**  
-`curl -O [URL]`  
+
+**To download and use these datasets, run:**  
+`curl -O [URL]`  
 `unzip [filename.zip]`  
 `cellxgene launch [filename.h5ad] --open`