doc/README: Add function Inputs guidelines

  For example:

  ```markdown

  # pkgs.coolFunction

  # pkgs.coolFunction {#pkgs.coolFunction}

  Description of what `coolFunction` does.

  `pkgs.coolFunction` *`name`* *`config`*

  ## Inputs

  Description of what `callPackage` does.

  `coolFunction` expects a single argument which should be an attribute set, with the following possible attributes:

  `name` (String)

  ## Inputs {#pkgs-coolFunction-inputs}

  If something's special about `coolFunction`'s general argument handling, you can say so here.

  Otherwise, just describe the single argument or start the arguments' definition list without introduction.

  *`name`* (String)

  : The name of the resulting image.

  `tag` (String; _optional_)

  *`config`* (Attribute set)

  : Introduce the parameter. Maybe you have a test to make sure `{ }` is a sensible default; then you can say: these attributes are optional; `{ }` is a valid argument.

  : Tag of the generated image.

    `outputHash` (String; _optional_)

    _Default:_ the output path's hash.

    : A brief explanation including when and when not to pass this attribute.

    : _Default:_ the output path's hash.

  ```

  Checklist:

  - Start with a synopsis, to show the order of positional arguments.

  - Metavariables are in emphasized code spans: ``` *`arg1`* ```. Metavariables are placeholders where users may write arbitrary expressions. This includes positional arguments.

  - Attribute names are regular code spans: ``` `attr1` ```. These identifiers can _not_ be picked freely by users, so they are _not_ metavariables.

  - _optional_ attributes have a _`Default:`_ if it's easily described as a value.

  - _optional_ attributes have a _`Default behavior:`_ if it's not easily described using a value.

  - Nix types aren't in code spans, because they are not code

  - Nix types are capitalized, to distinguish them from the camelCase [Module System](#module-system) types, which _are_ code and behave like functions.

#### Examples

To define a referenceable figure use the following fencing: