Created a new troubleshooting section in the migrations guide ...

First off, thanks to everyone for making these beautiful guides available!

I have created a section named "Troubleshooting" in migrations guide. I hope that is okay. Please see if this is per expectations from railsguides. I have generated the htmls, proof-read them, and they feel all right, but please review them and let me know if this was a good idea. If this is not a good fit, let me know and I'll remove it.

BTW, I based the example on Michael Lang's blog post:

Also, perhaps we need to mention somewhere that we also need w3c_validators gem to run the rake validat_guides task. The validation breaks as of now, with 37 errors. I don't think my change broke them, but I will double check it.

Best Regards, -Kedar (kedarmhaswade)