Improve installation guide

Complements d19f1d0.

Author: dr0i

README.md

@@ -12,16 +12,20 @@ The content pages can be found [in `/docs/`](/docs/)
 ## Installation
 If you are on debian derivates go install build-essentials:
 ```
- apt install build-essential
+apt install build-essential
 ```
 Then you can use the Ruby Dependency Management to build all you need:
 ```
- bundle install jekyll
- bundle install
+bundle install
 ```
 Start jekyll like this:
 ```
- bundle exec jekyll serve
+bundle exec jekyll serve
+```
+If you experience troubles, make sure to not have jekyll installed via you
+package manager. On debian derivates do:
+```
+apt purge jekyll
 ```
 
 ## Note