Add External Libraries

External-Libraries.md

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+CGround supports creating your own ground functions written in C. While theoretically any language that can compile to a shared object file can be used, C is the language most supported.
+
+# Creating an External Library (in C)
+For this example, we will make a function that returns "Hello, {arg1}!"
+
+Our ground syntax will be `call !Example_GreetUser $username &greet`
+
+## C code
+First, import `groundext.h`. Then, start a new function. For our example, we will call it `greetUser`.
+```c
+#include <groundext.h>
+
+GroundValue greetUser(GroundScope* scope, List args) {
+    
+}
+```
+To create our function, we need to access our arguments. Use `args.value`