Document compression support for pkg_tar

[dougthor42]

I didn't know this, but pkg_tar supports some compressions natively:

rules_pkg/pkg/private/tar/tar.bzl

Lines 81 to 95 in 8c7c2cf

	extension = ctx.attr.extension
	if extension and extension != "tar":
	compression = None
	dot_pos = ctx.attr.extension.rfind(".")
	if dot_pos >= 0:
	compression = ctx.attr.extension[dot_pos + 1:]
	else:
	compression = ctx.attr.extension
	if compression == "tgz":
	compression = "gz"
	if compression == "txz":
	compression = "xz"
	if compression:
	if compression in SUPPORTED_TAR_COMPRESSIONS:
	args.add("--compression", compression)

by setting the extension attribute:

pkg_tar(
    name = "package_tester_bin_tar",
    srcs = [":package_tester_bin"],
    include_runfiles = True,
    extension="gz",  # this bit!
)

Let's get that documented.