Write some sample plugins and document common pitfalls

[yifanlu]

There's problems people run into. We should keep a list of problems and solutions.

• Using newlib from plugins. Should use either SceLibc, sceClib* from SceLibKernel, or libk.
• Using user imports from kernel mode.
• Unsafe function usage in safe homebrew
• Need for mod.size = sizeof(mod)

Also the following samples should be created

• "Hello world" show FPS in game
• Port amphetamin
• Kernel module that exports user syscall/on demand loading