Been looking for a few days on this and I am not finding any good resources. I need to implement twitter bootstrap into my AEM 6.5 project and having a hard time finding any resources on how to go about this. Links are great and thanks!
I don't have an exact tutorial. How familiar are you with AEM Client libraries? There are a couple of approaches.
The simplest is to create a new client library and add the distributed Bootstrap CSS/JS and integrate that into your page. Part 3 of the WKND tutorial  should give you a decent understanding of how client libraries work and can be integrated into your page. The AEM We.Retail sample site actually uses bootstrap (although its an older version using LESS)
If you want to integrate Bootstrap and extend it using Sass it is possible to integrate it into the build process of your project. A similar approach is used by AEM's SPA editor..instead of React you would just be integrating an external Bootstrap project. This will be much more complicated and require some understanding of Maven and AEM project archetype. 
Lastly, you may want to align Bootstrap breakpoints with AEM's responsive grid. This is a decent blog post  and I believe the author is working on a more technical example...
Just an update in case others find this, there is a new blog post that can help customize the AEM Responsive Grid and integrate with Bootstrap breakpoints: https://medium.com/adobetech/customizing-the-aem-grid-3ff9c2121a36
Is it really "integrating" with bootstrap though?
As far as I can tell, following the article would actually create a different css that will "mock" the bootstrap grid and not really extend or integrate with it.