vweb: tmpl: add docs

vlib/vweb/tmpl/README.md

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+This package is to generate data-driven HTML output.
+
+# Directives
+Each directive begins with an `@` sign.
+Some directives begin contains a `{}` block, others only have `''` (string) parameters. More on the directives itself.
+
+Newlines on the beginning and end are ignored in `{}` blocks, otherwise this (see [if](##if) for this syntax):
+```html
+@if bool_val {
+    <span>This is shown if bool_val is true</span>
+}
+```
+would result in:
+```html
+
+    <span>This is shown if bool_val is true</span>
+
+```
+which could result in unreadable output.
+
+## if
+The if directive consists of three parts, the `@if` tag, the condition (same syntax like in V) and the `{}` block where you can write html which will be rendered if the condition is true:
+```
+@if <condition> {}
+```
+
+### Example
+```html
+@if bool_val {
+    <span>This is shown if bool_val is true</span>
+}
+```
+One-liner:
+```html
+@if bool_val { <span>This is shown if bool_val is true</span> }
+```
+
+The first example would result in:
+```html
+    <span>This is shown if bool_val is true</span>
+```
+while the one-liner results in:
+```html
+<span>This is shown if bool_val is true</span>
+```
+
+## for
+The for directive consists of three parts, the `@for` tag, the condition (same syntax like in V) and the `{}` block where you can write html which will be rendered for each loop:
+```
+@for <condition> {}
+```
+
+### Example
+```html
+@for i, val in my_vals {
+    <span>$i - $val</span>
+}
+```
+One-liner:
+```html
+@for i, val in my_vals { <span>$i - $val</span> }
+```
+
+The first example would result in:
+```html
+    <span>0 - "First"</span>
+    <span>1 - "Second"</span>
+    <span>2 - "Third"</span>
+    ...
+```
+while the one-liner results in:
+```html
+<span>0 - "First"</span>
+<span>1 - "Second"</span>
+<span>2 - "Third"</span>
+...
+```
+
+You can also write (and all other for condition syntaxes that are allowed in V):
+```html
+@for i = 0; i < 5; i++ {
+    <span>$i</span>
+}
+```
+
+## include
+The include directive is for including other html files (which will be processed as well) and consists of two parts, the `@include` tag and a following `'<path>'` string.
+The path parameter is relative to the `/templates` directory in the corresponding project.
+
+### Example
+Files
+```
+Project root
+/templates
+    - index.html
+    /headers
+        - base.html
+```
+
+
+`index.html`
+```html
+<div>@include 'header/base'</div>
+```
+> Note that there shouldn't be a file suffix, it is automatically appended and only allows `html` files.
+
+# Variables
+
+_In Progress_