Add naive make install for Linux

This enables a `sudo make install` that copies the build directory to
the PREFIX directory (/opt/OpenVR-AdvancedSettings/AdvancedSettings by
default).

The entire directory is being copied because Qt is incredibly weird
about how the INSTALLS variable works. From testing it appears that only
files that exist before the build process has started (even the
file_copy process) will be eligible for installation.

If the /OpenVR-AdvancedSettings/ directory is excluded, Qt will attempt
to remove the /opt/ folder on `make uninstall`, this is obviously not
desirable.

docs/building_for_linux.md

@@ -13,6 +13,7 @@
   * [Ubuntu 18.04 Bionic](#ubuntu-1804-bionic)
 - [Locations and Environment Variables](#locations-and-environment-variables)
 - [Building](#building)
+- [Installing](#installing)
 - [Contributing](#contributing)
 
 # Requirements
@@ -175,6 +176,12 @@ With the programs above installed and environment variables set, go into the roo
 
 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.
 
+# Installing
+
+The application can be installed by running `sudo make install` in the directory of the `Makefile`. 
+This will copy the build directory to `/opt/OpenVR-AdvancedSettings/AdvancedSettings` by default.
+The `PREFIX` location can be changed while running `qmake` by writing `qmake PREFIX=/your/dir/here`, then `make` and `sudo make install`.
+
 # Contributing
 
 For full details, see [CONTRIBUTING.md](CONTRIBUTING.md).