Add documentation

docs/src/getting_started.md

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+# Quickstart
+
+## Installation
+
+Building GRM currently requires the nightly Rust toolchain. The easiest way
+is using [`rustup`](https://rustup.rs/). Make sure that rustup is properly installed.
+
+Make sure that the nightly toolchain is installed:
+
+```
+$ rustup toolchain install nightly
+```
+
+```bash
+$ cargo +nightly install --git https://github.com/hakoerber/git-repo-manager.git --branch master
+```
+
+If you're brave, you can also run the development build:
+
+```bash
+$ cargo +nightly install --git https://github.com/hakoerber/git-repo-manager.git --branch develop
+```