Mention "upgrading 1.2.3 => 2.x" for haystack-search

[wfehr]

I've recently tried to update this app in my system and came across some errors/issues.

One with django-meta was fixed very fast (thanks again) with v2.0.5.

The other I just came across: I use haystack for blog-posts and with the newer version it won't work by default.
The reason, after going through the diff of this two versions (1.2.3, 2.0.5):

The file search_indexes.py was deleted.
I didn't find any doc mentioning this, the docs still state the same as before: installation.rst#enable-haystack-support

It would be nice to have something like a "how to update vom X to Y" or a updated block for this haystack-support.
Your app dropped the requirement for aldryn-search - which, well, is nice cause of deprecation - but the docs still indicate that code might work the same as before.

After recreating the search_indexes.py in my system, the search-page loads as expected.
Error message was following (from this line in the code in django-haystack):
No fields were found in any search_indexes. Please correct this before attempting to search.

[protoroto]

@wfehr Thansk for pointing this out. I'll discuss with @yakky what is the best approach on this issue.

[protoroto]

@wfehr As we dropped aldryn-search completely, probably the right thing to do is upgrade the docs and add a brief explaination on how to integrate haystack