[Docs] README doesn't explain how to install from main branch

[julian-risch]

https://github.com/deepset-ai/haystack#-installation doesn't mention how to install from main branch correctly but only from PyPI. We should adjust the text and add another installation example for the main branch (with extras), for example: pip install -e '.[all]'
or
pip install git+https://github.com/deepset-ai/haystack.git@main#egg=farm-haystack[colab,ocr]

[anakin87]

Hello!

I thought it was intentional because this option is also missing in the docs...

[julian-risch]

Hi Stefano :) I think we should also explain the currently missing alternative of how to install from the main branch. What's definitely wrong in the docs and the readme right now is the accompanying text saying "The following command installs the latest version of Haystack and all its dependencies from the main branch: ... pip install 'farm-haystack[all]'"

[julian-risch]

Cc @dfokina