Make C API build instructions usable

The Rust toolchain is not installed globally by default, so it is impossible to use
Cargo to install a library unless installing another toolchain for root, messing with
the root environment so the whole user toolchain can be located, or installing locally
and then manually moving the output. This fixes the installation instructions to use
the simplest of those solutions.

Author: rzumer

README.md

@@ -127,14 +127,15 @@ The resulting binary will not work on cpus that do not sport the same set of SIM
 
 > **NOTE** : You may use `rustc --print target-cpus` to check if the cpu is supported, if not `-C target-cpu=native` would be a no-op.
 
-### Building the C-API
+### Building the C API
 **rav1e** provides a C-compatible set of library, header and pkg-config file.
 
 To build and install it you can use [cargo-c](https://crates.io/crates/cargo-c):
 
 ```sh
-cargo install cargo-c
-cargo cinstall --release
+$ cargo install cargo-c
+$ cargo cinstall --destdir=$HOME/librav1e --release
+# mv $HOME/librav1e/* / && rmdir $HOME/librav1e
 ```
 
 ## Usage