Stored Procedures under Source Control, best practice
There are tools out there, such as this from Redgate, but I have always found that best is to save as SQL files, perhaps even in a Database Project (SSDT?) in your solution.  

Along with this, I suggest the following guidelines: 


Always assume the SVN version as the "current" / "latest"
Ensure that every script you run has an appropriate "if exists then drop" at the start
Remember to script your permissions, if any


You can initially create these SQL files by scripting directly from SSMS, and you can set SSMS to script all your "drop" and "create" as well as your permissions.