Add documentation

Documentation/tag-import.md

@@ -0,0 +1,48 @@
+# Tag: "Import"
+
+This tag loads and executes another template, but will **never** render any output. It's only purpose is for loading [macros](tag-macro.md) and [setting some variables](tag-set.md).
+
+## Node Information
+
+| Name      | Description            |
+|-----------|------------------------|
+| Tag Name  | `import`               |
+| End Tag   | N/A                    |
+| Rendering | Immediately; no output |
+
+| Parameter  | Description                                | 
+|------------|--------------------------------------------|
+| Template   | The name of the template you want to load. |
+
+The template name will depend on how your environment's loader is configured:
+
+- If you're using a file loader, template should be a valid file name.
+- If you're using a dictionary loader, template should be a key in that dictionary.
+
+## When to use it
+
+Should be used when you have some [macro](tag-macro.md)s or [set](tag-set.md)s that you want to reuse in multiple templates.
+
+Note that this tag may appear similar to the existing [include](https://stencil.fuller.li/en/latest/builtins.html#include) tag, but the purposes are opposite of each other:
+
+- `include` will render the included template, but never store changes to the context.
+- `import` will never render the imported template, but will store changes to the context.
+
+## Usage example
+
+```stencil
+{% import "common.stencil" %}
+
+{% call test "a" "b" "c" %}
+```
+
+`common.stencil` file:
+
+```stencil
+{% macro test a b c %}
+  Received parameters in test:
+  - a = "{{a}}"
+  - b = "{{b}}"
+  - c = "{{c}}"
+{% endmacro %}
+```

README.md

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 * [Set](Documentation/tag-set.md)
   * `{% set <Name> %}…{% endset %}`
   * Renders the nodes inside this block immediately, and stores the result in the `<Name>`  variable of the current context.
+* [Import](Documentation/tag-import.md)
+  * `{% import "common.stencil" %}`
+  * Imports any macro & set definitions from `common.stencil` into the current context.
 * [Map](Documentation/tag-map.md)
   * `{% map <Variable> into <Name> using <ItemName> %}…{% endmap %}`
   * Apply a `map` operator to an array, and store the result into a new array variable `<Name>` in the current context.