Add HTML output templates README.md (#371)

Describe the HTML Output Templates, how they are specified,
and placeholder usage within these templates.

src/wireviz/templates/README.md

@@ -0,0 +1,51 @@
+# HTML Output Templates
+
+This is the standard folder where WireViz looks for an HTML output template file.
+
+## Which HTML Output Template File is Used?
+
+A named HTML output template can optionally be specified as
+`metadata.template.name` in the YAML input:
+```yaml
+metadata:
+  template:
+    name: din-6771
+```
+In the case above, WireViz will search for a template file named
+`din-6771.html` in these folders:
+1. In the same folder as the YAML input file.
+2. In this standard template folder.
+
+If no HTML output template is specified, the `simple` template is assumed
+(i.e. filename `simple.html`, and in this case,
+only the standard template folder is searched).
+
+## Placeholders in HTML Output Templates
+
+HTML output template files might contain placeholders that will be replaced by
+generated text by WireViz when producing HTML output based on such a template.
+A placeholder starts with `<!-- %`, followed by a keyword, and finally `% -->`.
+Note that there must be one single space between `--` and `%` at both ends.
+
+| Placeholder | Replaced by |
+| --- | --- |
+| `<!-- %generator% -->` | The application name, version, and URL |
+| `<!-- %fontname% -->`  | The value of `options.fontname` |
+| `<!-- %bgcolor% -->`   | The HEX color translation of `options.bgcolor` |
+| `<!-- %filename% -->`  | The output path and filename without extension |
+| `<!-- %filename_stem% -->` | The output filename without path nor extension |
+| `<!-- %bom% -->`           | BOM as HTML table with headers at top |
+| `<!-- %bom_reversed% -->`  | Reversed BOM as HTML table with headers at bottom |
+| `<!-- %sheet_current% -->` | `1` (multi-page documents not yet supported) |
+| `<!-- %sheet_total% -->`   | `1` (multi-page documents not yet supported) |
+| `<!-- %diagram% -->`       | Embedded SVG diagram as valid HTML |
+| `<!-- %diagram_png_base64% -->` | Embedded base64 encoded PNG diagram as URI |
+| `<!-- %{item}% -->`             | String or numeric value of `metadata.{item}` |
+| `<!-- %{item}_{i}% -->`         | Category number `{i}` within dict value of `metadata.{item}` |
+| `<!-- %{item}_{i}_{key}% -->`   | Value of `metadata.{item}.{category}.{key}` |
+
+Note that `{item}`, `{category}` and `{key}` in the description above can be
+any valid YAML key, and `{i}` is an integer representing the 1-based index of
+category entries in a dict `metadata.{item}` entry.
+The `{` and `}` characters are not literally part of the syntax, just used in
+this documentation to enclose the variable parts of the keywords.