Clarify Oodle instructions, link against "oodle" directory

While you can just drop the Oodle static library into /usr, I'm not a
big fan of doing it, especially when the binary didn't come from my
machine or package manager. You can set a RUSTFLAGS environment variable
or cargo config file to specify a link path, but having the build script
search there for you is easier.

The docs now specify which variant of Oodle is required, since there are
multiple. It also specifically mentions the static library, because I
had some weird issues leaving the dynamic library in that folder, where
the executables wouldn't start unless it could dynamically load the .so.

.gitignore

@@ -1,4 +1,5 @@
 /target
+/oodle
 .idea/
 config.json
 *.bin

USAGE.md

@@ -11,7 +11,7 @@ Kawari is designed to be easy to run, with the goal of being accessible to anyon
 ## Prerequisites
 * Linux
 * Copy of the game updated to the supported game version (see README)
-* Oodle (can be obtained from [here](https://github.com/WorkingRobot/OodleUE))
+* Oodle Network Compression (can be obtained from [here](https://github.com/WorkingRobot/OodleUE), place static library (`.a`) into `oodle` directory)
 
 ## Setup
 

build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("cargo:rustc-link-search=./oodle");
+}