Document how to compile plugins on linux

misc/plugin-dev.md

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+This document details how to compile a basic plugin in Visual Studio, MinGW, or GCC.
+
+To find the functions and variables available for use in plugins, look for CC_API/CC_VAR in the .h files.
+
+[Source code of some actual plugins](https://github.com/UnknownShadow200/ClassiCube-Plugins/tree/master/src)
+
+### Setup
+
+You need to download and install either Visual Studio, MinGW, or GCC.
+
+*Note: MinGW/GCC are relatively small, while Visual Studio is gigabytes in size.  
+If you're just trying to compile a plugin on Windows you might want to use MinGW. See main readme.*
+
+I assume your directory is structured like this:
+```
+src/...
+TestPlugin.c
+```
+Or in other words, in a directory somewhere, you have a file named ```TestPlugin.c```, and then a sub-directory named ```src``` which contains the game's source code.
+
+### Basic plugin
+```C
+#include "src/Chat.h"
+#include "src/GameStructs.h"
+
+static void TestPlugin_Init(void) {
+        String msg = String_FromConst("Hello world!");
+        Chat_Add(&msg);
+}
+
+int Plugin_ApiVersion = 1;
+struct IGameComponent Plugin_Component = { TestPlugin_Init };
+```
+Here's the idea for a basic plugin that shows "Hello world" in chat when the game starts. Alas, this won't compile...
+
+### Basic plugin boilerplate
+```C
+#include "src/Chat.h"
+#include "src/GameStructs.h"
+
+#ifdef CC_BUILD_WIN
+    #define CC_API __declspec(dllimport)
+    #define CC_VAR __declspec(dllimport)
+    #define EXPORT __declspec(dllexport)
+#else
+    #define CC_API
+    #define CC_VAR
+    #define EXPORT __attribute__((visibility("default")))
+#endif
+
+static void TestPlugin_Init(void) {
+        String msg = String_FromConst("Hello world!");
+        Chat_Add(&msg);
+}
+
+EXPORT int Plugin_ApiVersion = 1;
+EXPORT struct IGameComponent Plugin_Component = { TestPlugin_Init };
+```
+With this boilerplate, we're ready to compile the plugin.
+All plugins require this boilerplate, so feel free to copy and paste it.
+
+---
+
+### Linux
+#### For Linux
+**Compiling**
+
+```gcc TestPlugin.c -o TestPlugin.so -shared -fPIC```
+
+Then put ```TestPlugin.so``` into your game's ```plugins``` folder. Done.
+
+#### Cross compile for Windows 32 bit
+**Prerequisites:**
+
+1) Compile the game, see ```Cross compiling for windows``` in main readme
+2) Rename compiled executable to ClassiCube.exe
+3) Install the ```mingw-w64-tools``` package
+
+**Compiling**
+
+First, generate list of exported symbols: 
+
+```gendef src/ClassiCube.exe```
+
+Next create a linkable library: 
+
+```i686-w64-mingw32-dlltool -d ClassiCube.def -l libClassiCube.a -D ClassiCube.exe```
+
+Finally compile the plugin:
+
+```i686-w64-mingw32-gcc TestPlugin.c -o TestPlugin.dll -s -shared -L . -lClassiCube```
+
+Then put ```TestPlugin.dll``` into your game's ```plugins``` folder. Done.
+
+##### Ultra small dlls
+If you **ONLY** use code from the game (no external libraries and no C standard library functions), add ```-nostartfiles -Wl,--entry=0``` to the compile flags
+
+This step isn't necessary, the dll works fine without it. But it does reduce the size of the dll from 11 to 4 kb.
+
+#### Cross compile for Windows 64 bit
+Repeat the steps of *Cross compile for Windows 32 bit*, but use ```x86_64-w64-mingw32``` instead of ```i686-w64-mingw32```
+
+
+### Windows
+TODO when I have some more time...
+