Update the instruction not to install via crates.io

crates.io's version is outdated and we haven't figured out the release process yet.
cc rust-lang#168

README.md

@@ -33,20 +33,13 @@ Then you can do:
 
 ```sh
 $ rustup component add rustc-dev llvm-tools-preview --toolchain nightly-2020-11-19
-$ cargo +nightly-2020-11-19 install semverver
+$ cargo +nightly-2020-11-19 install --git https://github.com/rust-lang/rust-semverver
 ```
 
 You'd also need `cmake` for some dependencies, and a few common libraries (if you hit
 build failures because of missing system-wide dependencies, please open an issue, so they
 can be added here).
 
-You can also install the newest version of the tool from git:
-
-```sh
-$ rustup component add rustc-dev llvm-tools-preview --toolchain nightly-2020-11-19
-$ cargo +nightly-2020-11-19 install --git https://github.com/rust-lang/rust-semverver
-```
-
 <details>
 
 <summary>