Create a compilation guide

COMPILING.md

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+# Linux
+### 1. Requirements:
+Mono >= 5.4.0 (>= 5.8.0 recommended)
+Please check [here](http://www.mono-project.com/download/) for stable or [here](http://www.mono-project.com/download/alpha/) for an alpha release.  
+NuGet >= 4.4.0  
+msbuild  
+git
+
+### 2. Cloning project
+Clone the entire repository with submodules using
+```
+git clone https://github.com/ppy/osu --recursive
+```
+Then restore NuGet packages from the repository
+```
+nuget restore
+```
+We also need OpenTK >= 3.0.0-pre
+```
+nuget install opentk -version=3.0.0-pre
+```
+### 3. Compiling
+Simply run `msbuild` where `osu.sln` is located, this will create all binaries in `osu/osu.Desktop/bin/Debug`.
+### 4. Optimizing
+If you want additional performance you can change build type to Release with
+```
+msbuild -p:Configuration=Release
+```
+Additionally, mono provides an AOT utility which attempts to precompile binaries. You can utilize that by running
+```
+mono --aot ./osu\!.exe
+```
+### 5. Troubleshooting
+You may run into trouble with NuGet versioning, as the one in packaging system is almost always out of date. Simply run 
+```
+nuget
+sudo nuget update -self
+```
+**Warning** NuGet creates few config files when it's run for the first time.
+Do not run NuGet as root on the first run or you might run into very peculiar issues.