Update Ninject.sln to reference README.md

After the rename of README.markdown to README.md (f884f9998), the file's link in the solution file was orphaned leading to an error when trying to open it.

![image](https://cloud.githubusercontent.com/assets/1066014/4799220/cfd81836-5e1a-11e4-8919-d640f9a8b8df.png)