[README] Wrong use of ember serve command

[belgoros]

In README Exampl App section, the line about installing and starting the app with ember serve is wrong:

git clone https://github.com/simplabs/ember-simple-auth.git
cd ember-simple-auth
yarn install && ember serve

and raises the error:

You should CD into packages/test-app before running ember serve command:

➜  ember-simple-auth git:(master) cd packages/test-app 
➜  test-app git:(master) ember s
...
Build successful (16411ms) – Serving on http://localhost:4200/



Slowest Nodes (totalTime => 5% )              | Total (avg)         
----------------------------------------------+---------------------
Babel: ember-bootstrap (1)                    | 3015ms              
Babel: @ember/test-helpers (2)                | 1712ms (856 ms)     
Babel: ember-test-helpers (2)                 | 1463ms (731 ms)     
Babel: @ember-data/store (2)                  | 909ms (454 ms)      
ember-auto-import-analyzer (5)                | 824ms (164 ms)   
...