Clarify that setup options are exclusive (#185)

Doing SciPy 2020 tutorial that is happening now, some users got confused about the
setup instructions and ended up following more than one of the options.

This commit clarifies that only one of those are necessary

Author: fbidu

README.md

@@ -29,13 +29,15 @@ schedulers (odd sections.)
 
 ## Prepare
 
-You should clone this repository
+1. You should clone this repository
 
     git clone http://github.com/dask/dask-tutorial
 
 and then install necessary packages.
+There are three different ways to achieve this, pick the one that best suits you.
+They are, in order of preference: 
 
-#### a) Create a conda environment (preferred)
+#### 2a) Create a conda environment (preferred)
 
 In the main repo directory
 
@@ -44,7 +46,7 @@ In the main repo directory
     jupyter labextension install @jupyter-widgets/jupyterlab-manager
     jupyter labextension install @bokeh/jupyter_bokeh
 
-#### b) Install into an existing environment
+#### 2b) Install into an existing environment
 
 You will need the following core libraries
 
@@ -57,7 +59,7 @@ You may find the following libraries helpful for some exercises
 Note that this options will alter your existing environment, potentially changing the versions of packages you already 
 have installed. 
 
-#### c) Use Dockerfile
+#### 2c) Use Dockerfile
 
 You can build a docker image out of the provided Dockerfile.
 
@@ -71,6 +73,8 @@ The above command will give an URL (`Like http://(container_id or 127.0.0.1):888
 can be used to access the notebook from browser. You may need to replace the given hostname with "localhost" or
 "127.0.0.1".
 
+**You should follow only one of the options above!**
+
 ### Launch notebook
 
 From the repo directory