Update EasyRPG Player core documentation

[Mimigris]

• Mention that lzh files can be opened (recent feature, already set as valid in the libretro info file)
• Clarify that the RTPs must be placed in the system folder set for libretro and not a hardcoded system folder
• Mention that games can write in arbitrary files from the directory of the game (feature that can be used by some games to e.g. write data in txt files)
• Add info on where Soundfont, Font and WildMidi are to be placed in the frontend's system directory

[Mimigris]

The spellcheck seems to come from files unrelated to this pull request that I haven't touched from what I can see, so no clue why it complains here.

[hizzlekizzle]

Yeah, don't worry about the spell check. These changes look good to me.