Create intro.md

before we can add advanced examples, we need type inference: just because I declare a variable as `[]` doesn't mean I'll never write a value to it.

docs/intro.md

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+# Hello, world!
+
+Welcome to mers! Start by creating a file for your mers code:
+
+    println("Hello, world!")
+
+now run it: `mers <file>` (replace mers the location of the compiled executable if it's not in your PATH)
+
+# basic concepts
+
+## variables
+
+    a = 15
+    println("a: " + a.to_string())
+    // clones the value
+    b = a
+    a = 25
+    println("a: " + a.to_string())
+    println("b: " + b.to_string())
+
+## if statements
+
+    // type: bool
+    condition = true
+    if condition {
+        println("yes")
+    } else {
+        println("no")
+    }
+
+### else-if statements
+
+    number = 15
+    if number == 10 {
+        println("ten")
+    } else if number == 15 {
+        println("fifteen")
+    } else {
+        println("another number")
+    }
+
+## switch statements
+
+    condition = true
+    // type: string/int
+    val = if condition {
+        "some text"
+    } else {
+        15
+    }
+    // do different things depending on the type
+    switch! val {
+        string println("text: " + val)
+        int {
+            // we need to convert val to a string before we can use it here
+            println("number: " + val.to_string())
+        }
+    }
+
+## match statements
+
+\[...]