Add config file to .gitignore

We want to make sure people don't accidentally commit their config files when opening pull-requests and such
Adds instructions to README for Docker setup

From my investigation using a bash session on the Dockerfile, cloudtunes-server never started with superviser because it would always fail with:

```
!!!Cannot import local settings!!!

You need to copy cloudtunes/settings/local.example.py
to cloudtunes/settings/local.py and fill in the None's.

https://github.com/jakubroztocil/cloudtunes#installation
```
Add Dockerfile step to change config file
Already excluded in cloudtunes-server/.gitignore
Removing instructions for api settings...

Was already added in https://github.com/jakubroztocil/cloudtunes/commit/6ddb15d80e7fa383aec7533ed5b7bd92d8969c94
Merge pull request #20 from petems/add_dockerfile_instructions

Add instructions to get Dockerfile working