add functions docs

functions.md

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+# Functions
+
+Functions are first-class citizens in ginger. The simplest anonymous function
+can be created like so:
+
+```
+(. fn [x]
+    (: + x 1))
+```
+
+This function takes in a single number and returns that number plus one. It
+could be assigned to a variable on the package using `def`, or by using the
+shortcut `defn`. The following two statements are equivalent:
+
+```
+
+(def incr
+    (. fn [x]
+        (: + x 1)))
+
+(. defn incr [x]
+    (: + x 1))
+```
+
+## Variadic function
+
+Functions may take in any number of variables using the `...` syntax, similar to
+how go does variadic functions. The variadic variable must be the last in the
+function's argument list, and is used as a list inside of the function. A list
+may also be used as the input for a variadic function, also using the `...`
+syntax.
+
+The following is an example of both defining a variadic function and both ways
+of using it. `+` is a variadic function in this example:
+
+```
+(. defn avg [...x]
+    (/
+        (+ x...)
+        (len x)))
+
+(: fmt.Println (avg 1 2 3 4 5))
+```