Developers have been using this Spotlight trick for quite some time to search their source code.
Add the file types you want to Spotlight RichText.mdimporter info.plist /System/Library/Spotlight/RichText.mdimporter
info.plist
<string>public.swift-source</string>
<string>public.objective-c-source</string>
Terminal
mdimport -r /System/Library/Spotlight/RichText.mdimporter
sudo mdutil -E
But access is now restricted in El Capitan. Saving the info.plist is not possible.
Anyone have a solution?
Boot to Recovery HD and then disable SIP in terminal.app
csrutil disable
If you want to enable all protections except a few, you can poke holes in the protection to allow debugging, dtrace, modifying filesystem restricted files, etc..
csrutil enable -without fs
See this nice answer by Rich Trouton as well as his blog for details on SIP. I can't find official Apple documentation about these options - perhaps WWDC 2016 will have new documentation on how developers can relax the default security to modify things and run custom kext, etc...
In short - once you disable or enable without the fs restrictions, you can then edit the files needed to change the spotlight indexing rules for RichText as before.