Add table of contents and reduce indentation of headers

The #### headers do not look very distinct in Github markup, reducing
the header indentation will make them stand out more.

docs/building_for_linux.md

@@ -1,6 +1,20 @@
-# Building on Linux
-## Requirements
-### Compiler and Build Essentials
+- [Requirements](#requirements)
+  * [Compiler and Build Essentials](#compiler-and-build-essentials)
+  * [Qt](#qt)
+  * [Official Qt Installer](#official-qt-installer)
+  * [Unofficial Ubuntu Packages](#unofficial-ubuntu-packages)
+  * [`qtchooser` and versions](#-qtchooser--and-versions)
+  * [X11](#x11)
+  * [`clang-tidy` and `bear`](#-clang-tidy--and--bear-)
+- [TL;DR: for Ubuntu](#tl-dr--for-ubuntu)
+  * [Ubuntu 16.04 Xenial](#ubuntu-1604-xenial)
+  * [Ubuntu 18.04 Bionic](#ubuntu-1804-bionic)
+- [Locations and Environment Variables](#locations-and-environment-variables)
+- [Building](#building)
+- [Contributing](#contributing)
+
+# Requirements
+## Compiler and Build Essentials
 
 You will need either `clang` og `g++`. At least `g++-7` or `clang` version `5.0` is required due to C++17 support.
 You will additionally need the `build-essential` and `libgl1-mesa-dev` packages.
@@ -11,15 +25,15 @@ sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo apt update
 sudo apt install g++-7
 ```
-### Qt
+## Qt
 
 At least version `5.12` is required.
 
-#### Official Qt Installer
+## Official Qt Installer
 
 The easiest way to get it is from the [official Qt installer](https://www.qt.io/download-qt-installer).
 
-#### Unofficial Ubuntu Packages
+## Unofficial Ubuntu Packages
 
 If you don't want to use the offical installer, you can use [this](https://launchpad.net/~beineri) PPA.
 
@@ -39,7 +53,7 @@ sudo apt install qt512-meta-full
 ```
 If you choose the wrong Ubuntu version you will get an error from `apt` about there not being a `Release` file. Try the other one instead.
 
-#### `qtchooser` and versions
+## `qtchooser` and versions
 
 If you have multiple Qt installations you might need to select the correct one using `qtchooser`.
 
@@ -80,11 +94,11 @@ qtchooser -install opt-qt512 /opt/qt512/bin/qmake
 
 Afterwards you should set the `QT_SELECT` environment variable to the name you chose (we used `opt-qt512`).
 
-### X11
+## X11
 
 X11 packages are currently needed for sending keystrokes to the desktop from VR. Install the packages with `sudo apt-get install libx11-dev libxt-dev libxtst-dev`. This feature will be gated behind a compile flag soon.
 
-### `clang-tidy` and `bear`
+## `clang-tidy` and `bear`
 
 In order to use `clang-tidy` you will need `bear clang-tidy` in addition to the above (and `clang`). You will only need this if you're going to make contributions to the codebase.
 
@@ -92,8 +106,8 @@ In order to use `clang-tidy` you will need `bear clang-tidy` in addition to the
 sudo apt install bear clang-tidy
 ```
 
-## TL;DR: for Ubuntu
-### Ubuntu 16.04 Xenial
+# TL;DR: for Ubuntu
+## Ubuntu 16.04 Xenial
 ```bash
 sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo add-apt-repository ppa:beineri/opt-qt-5.12.2-xenial
@@ -106,7 +120,7 @@ sudo apt-get install bear clang-tidy
 qtchooser -install opt-qt512 /opt/qt512/bin/qmake
 ```
 
-### Ubuntu 18.04 Bionic
+## Ubuntu 18.04 Bionic
 ```bash
 sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo add-apt-repository ppa:beineri/opt-qt-5.12.2-bionic
@@ -119,7 +133,7 @@ sudo apt-get install bear clang-tidy
 qtchooser -install opt-qt512 /opt/qt512/bin/qmake
 ```
 
-## Locations and Environment Variables
+# Locations and Environment Variables
 
 The following environmental variables are relevant for building the project.
 
@@ -130,12 +144,12 @@ The following environmental variables are relevant for building the project.
 
 If an environment variable isn't set a default value will be provided. The default values are shown in the table below.
 
-## Building
+# Building
 
 With the programs above installed and environment variables set, go into the root folder of the repository and run `./build_scripts/linux/build_linux.sh`.
 
 If you copy the `third-party/openvr/lib/linux64/libopenvr_api.so` file into `/lib` you can run the `AdvancedSettings` file without anything else. Otherwise you'll need to run the `run-with-library.sh` file.
 
-## Contributing
+# Contributing
 
 For full details, see [CONTRIBUTING.md](CONTRIBUTING.md).