Add Rhai book.

doc/src/language/modules/import.md

@@ -0,0 +1,45 @@
+Import a Module
+===============
+
+{{#include ../../links.md}}
+
+A module can be _imported_ via the `import` statement, and its members are accessed via '`::`' similar to C++.
+
+```rust
+import "crypto" as lock;        // import the script file 'crypto.rhai' as a module named 'lock'
+
+lock::encrypt(secret);          // use functions defined under the module via '::'
+
+lock::hash::sha256(key);        // sub-modules are also supported
+
+print(lock::status);            // module variables are constants
+
+lock::status = "off";           // <- runtime error - cannot modify a constant
+```
+
+`import` statements are _scoped_, meaning that they are only accessible inside the scope that they're imported.
+They can appear anywhere a normal statement can be, but in the vast majority of cases `import` statements are
+group at the beginning of a script.
+
+It is, however, not advised to deviate from this common practice unless there is a _Very Good Reason™_.
+Especially, do not place an `import` statement within a loop; doing so will repeatedly re-load the same module
+during every iteration of the loop!
+
+```rust
+let mod = "crypto";
+
+if secured {                    // new block scope
+    import mod as c;            // import module (the path needs not be a constant string)
+
+    c::encrypt(key);            // use a function in the module
+}                               // the module disappears at the end of the block scope
+
+crypto::encrypt(others);        // <- this causes a run-time error because the 'crypto' module
+                                //    is no longer available!
+
+for x in range(0, 1000) {
+    import "crypto" as c;       // <- importing a module inside a loop is a Very Bad Idea™
+
+    c.encrypt(something);
+}
+```