Add note about expected libraryproduct extensions per platform?

[IanButterworth]

I just came across a library that built .so files on MacOS, and didn't realize that these were being rejected by:

BinaryProvider.jl/src/PlatformNames.jl

Lines 500 to 508 in 241bdc4

	function parse_dl_name_version(path::AbstractString, platform::Platform)
	dlext_regexes = Dict(
	# On Linux, libraries look like `libnettle.so.6.3.0`
	"so" => r"^(.*?).so((?:\.[\d]+)*)$",
	# On OSX, libraries look like `libnettle.6.3.dylib`
	"dylib" => r"^(.*?)((?:\.[\d]+)*).dylib$",
	# On Windows, libraries look like `libnettle-6.dylib`
	"dll" => r"^(.*?)(?:-((?:[\.\d]+)*))?.dll$"
	)

It was difficult to debug because there was no warning. Perhaps it could be helpful to explain that requirement, either in the readme and/or LibraryProduct doc notes (which as they are, reinforced to me that a .so file should be discoverable):

BinaryProvider.jl/src/Products.jl

Lines 50 to 58 in 229e2c1

	"""
	A `LibraryProduct` is a special kind of `Product` that not only needs to exist,
	but needs to be `dlopen()`'able. You must know which directory the library
	will be installed to, and its name, e.g. to build a `LibraryProduct` that
	refers to `"/lib/libnettle.so"`, the "directory" would be "/lib", and the
	"libname" would be "libnettle". Note that a `LibraryProduct` can support
	multiple libnames, as some software projects change the libname based on the
	build configuration.
	"""