Reloc addrs docs

[AngheloAlf]

Add documentation for the reloc_addrs.txt format.

I also changed create_config to create a basic symbol_addrs.txt and reloc_addrs.txt from the information it is already gathering.

The generated symbol_addrs.txt file just names the entrypoint and the main symbols.
Example for the generated file:

// Visit https://github.com/ethteck/splat/wiki/Adding-Symbols for documentation about this file
entrypoint = 0x80000400; // type:func
main = 0x80000520; // type:func

The reloc_addrs.txt file contains relocs for the bss symbols referenced by the entrypoint.
It also contains a commented out entry for the stack, it is commented out because we can't infer the actual start of that symbol with just the info on the entrypoint, instead I left a comment explaining this.
Example for the generated file:

// Visit https://github.com/ethteck/splat/wiki/Advanced-Reloc for documentation about this file
// entrypoint relocs
rom:0x001000 reloc:MIPS_HI16 symbol:main_BSS_START
rom:0x001004 reloc:MIPS_LO16 symbol:main_BSS_START

rom:0x001008 reloc:MIPS_HI16 symbol:main_BSS_SIZE
rom:0x00100C reloc:MIPS_LO16 symbol:main_BSS_SIZE

// This entry corresponds to the "stack top", which is the end of the array used as the stack for the main segment.
// It is commented out because it was not possible to infer what the start of the stack symbol is, so you'll have to figure it out by yourself.
// Once you have found it you can properly name it and specify the length of this stack as the addend value here.
// The address of the end of the stack is 0x801AD9A0.
// A common size for this stack is 0x2000, so try checking for the address 0x801AB9A0. Note the stack may have a different size.
// rom:0x001030 reloc:MIPS_HI16 symbol:main_stack addend:0xXXXX
// rom:0x001038 reloc:MIPS_LO16 symbol:main_stack addend:0xXXXX

closes #465