README: document when libgit2 is statically linked

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

The text follows my understanding of https://github.com/rust-lang/git2-rs/blob/f1f09cee7b332d2b494d14d46fd2ec8e5203916a/libgit2-sys/build.rs#L25-L56. 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.

README.md

@@ -19,12 +19,17 @@ stable release as well.
 Currently this library requires libgit2 1.7.2 (or newer patch versions). The
 source for libgit2 is included in the libgit2-sys crate so there's no need to
 pre-install the libgit2 library, the libgit2-sys crate will figure that and/or
-build that for you.
+build that for you. On the other hand, if an appropriate version of `libgit2`
+is present, `git2` will attempt to dynamically link it.
 
-You can enable the Cargo feature `vendored-libgit2` to always compile and
-statically link to a copy of libgit2. Sometimes the libgit2 on the system is
-required to be found and used even when `vendored-libgit2` is activated. In
-this case, you shall set the environment variable `LIBGIT2_NO_VENDOR=1`.
+To be more precise, the vendored `libgit2` is linked statically if two
+conditions both hold:
+
+- The enviornment variable `LIBGIT2_NO_VENDOR=1` is **not** set
+- **and** either a) The Cargo feature `vendored-libgit2` is set or b) an
+  appropriate version of `libgit2` cannot be found on the system.
+
+In particular, note that the environment variable overrides the Cargo feature.
 
 ## Building git2-rs