README: document when libgit2 is statically linked

[ilyagr]

I found the previous paragraph in the current version of the README quite confusing.

The text follows my understanding of

git2-rs/libgit2-sys/build.rs

Lines 25 to 56 in f1f09ce

	let https = env::var("CARGO_FEATURE_HTTPS").is_ok();
	let ssh = env::var("CARGO_FEATURE_SSH").is_ok();
	let vendored = env::var("CARGO_FEATURE_VENDORED").is_ok();
	let zlib_ng_compat = env::var("CARGO_FEATURE_ZLIB_NG_COMPAT").is_ok();
	// Specify `LIBGIT2_NO_VENDOR` to force to use system libgit2.
	// Due to the additive nature of Cargo features, if some crate in the
	// dependency graph activates `vendored` feature, there is no way to revert
	// it back. This env var serves as a workaround for this purpose.
	println!("cargo:rerun-if-env-changed=LIBGIT2_NO_VENDOR");
	let forced_no_vendor = env::var_os("LIBGIT2_NO_VENDOR").map_or(false, |s| s != "0");
	if forced_no_vendor {
	if try_system_libgit2().is_err() {
	panic!(
	"\
	The environment variable `LIBGIT2_NO_VENDOR` has been set but no compatible system libgit2 could be found.
	The build is now aborting. To disable, unset the variable or use `LIBGIT2_NO_VENDOR=0`.
	",
	);
	}
	// We've reached here, implying we're using system libgit2.
	return;
	}
	// To use zlib-ng in zlib-compat mode, we have to build libgit2 ourselves.
	let try_to_use_system_libgit2 = !vendored && !zlib_ng_compat;
	if try_to_use_system_libgit2 && try_system_libgit2().is_ok() {
	// using system libgit2 has worked
	return;
	}

. I hope it's OK to hide the bit about zlib under the case where an "appropriate version of libgit2 cannot be found". Feel free to edit this further.

Cc: commit 59a81ca

I looked into this because I was confused that, after upgrading the Homebrew version of libgit2, my builds of https://github.com/martinvonz/jj started linking libgit2 statically. I'm planning to try enabling vendored_libgit2 in that project permanently and recommending that people use the environment variable LIBGIT2_NO_VENDOR=1 if they want dynamic linking.

[ehuss]

Thanks!