improve first impression after installing waliki

[humrochagf]

I was following the get started guide from README.md and got some errors at the process that can be warned to avoid frustration on new users and a better first impression:

1 - error at the attachment plugin

The attachment plugin have a pre requisite on django-sendfile that is discribed inside it's docs but is not mentioned on startup and leads new users to a import error and to spend some time figuring out whats happening.

2 - markdown dependency

My first action after installation was on creating a markdown page, but it generates another error that could be avoided if warned at the startup guide.

I think that's all, and thanks for this awesome project 😄

[mgaitan]

thanks for your feedback @humrochagf. I'll tackle it in my next solo-sprint, if no PR come before.