Add plugin:newFileFormat to the documentation

[bbrainstormer]

plugin:newFileFormat is currently not documented anywhere.

As seen here, for it accepts a table with the following fields:

• binary (boolean): whether or not the file format is binary or not
• supports (integer): Some sort of bitfield I assume, but I'm not sure what each one represents. Appears to be used to set the CustomFormatDefinition.flags variable; I can look into it later
• extensions/extension (string or string[]): All valid file extensions
• name (string): The name of the file format, I assume
• onload (function) and onsave (function): Determine behavior on load and save

[bbrainstormer]

Here are the flags (found in file_format.h) :

#define FILE_SUPPORT_LOAD               0x00000001
#define FILE_SUPPORT_SAVE               0x00000002
#define FILE_SUPPORT_RGB                0x00000004
#define FILE_SUPPORT_RGBA               0x00000008
#define FILE_SUPPORT_GRAY               0x00000010
#define FILE_SUPPORT_GRAYA              0x00000020
#define FILE_SUPPORT_INDEXED            0x00000040
#define FILE_SUPPORT_LAYERS             0x00000080
#define FILE_SUPPORT_FRAMES             0x00000100
#define FILE_SUPPORT_PALETTES           0x00000200
#define FILE_SUPPORT_SEQUENCES          0x00000400
#define FILE_SUPPORT_GET_FORMAT_OPTIONS 0x00000800
#define FILE_SUPPORT_TAGS               0x00001000
#define FILE_SUPPORT_BIG_PALETTES       0x00002000 // Palettes w/more than 256 colors
#define FILE_SUPPORT_PALETTE_WITH_ALPHA 0x00004000
#define FILE_ENCODE_ABSTRACT_IMAGE      0x00008000 // Use the new FileAbstractImage
#define FILE_GIF_ANI_LIMITATIONS        0x00010000

No idea if they are documented somewhere else