docs: Add extra install examples for backends (#122)

* Add install example commands for the 'local' and 'reana' extras when
  using the respective backends.

README.md

@@ -51,6 +51,10 @@ Commands:
 
 Running the example from the [ATLAS Exotics Rome Workshop 2018][ATLAS Exotics Workshop 2018] using the `local` backend:
 
+```
+python -m pip install 'recast-atlas[local]'
+```
+
 ```
 recast run examples/rome --backend local
 ```
@@ -59,6 +63,10 @@ recast run examples/rome --backend local
 
 Running the example from the [ATLAS Exotics Rome Workshop 2018][ATLAS Exotics Workshop 2018] using the `reana` backend:
 
+```
+python -m pip install 'recast-atlas[reana]'
+```
+
 ```
 recast run examples/rome --backend reana
 ```