Add README file.

README.md

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+# Astroneer Modloader
+
+A modloader for Astroneer, rewritten in Rust.
+
+## Installation
+
+### Windows
+
+Download the modloader (`astro_modloader.exe`) from the [releases
+page](https://github.com/AstroTechies/astro_modloader/releases/latest), below the changelog.
+
+### Linux
+
+Pre-built binaries are not yet dsitributed for Linux. To build the modloader yourself on Linux:
+
+- Use your distribution's package manager to install `rustup`, `git`, and `build-essential`,
+- Use `rustup` to install the Rust programming language,
+- Use `cargo` to install `cargo-about`,
+- Use `git` to clone the modloader's repository,
+- Then run the following commands in the root of the repository:
+
+```
+export USE_PRECOMPILED_CPP_LOADER=1
+export USE_PREBUILT_ASSETS=1
+cargo build --release
+```
+
+The last command may take a while to run. Once it's done, the executable (`astro_modloader`) will be
+in `target/release`.