readme: explain that Rust >= 1.60 is needed
We have `rust-version = "1.60"` in `Cargo.toml`, but Cargo doesn't
check that until after dependency resolution, which fails if you have
an older version (I don't know why).
@@ -145,6 +145,13 @@ See below for how to build from source. There are also
[pre-built binaries](https://github.com/martinvonz/jj/releases) for Windows,
Mac, or Linux (musl).
+If you're installing from source, you need to use Rust version 1.60 or higher,
+or you will get a cryptic message like this:
+```
+error: failed to select a version for the requirement `libgit2-sys = "=0.14.0"``
+candidate versions found which didn't match: 0.13.2+1.4.2, 0.13.1+1.4.2, 0.13.0+1.4.1, ...
+```
+
### Linux
On most distributions, you'll need to build from source using `cargo` directly.
