docs: list unirate_api in provider tables

Exchanger merged UniRateAPI support in florianv/exchanger#180. Swap
auto-inherits provider services from Exchanger's Registry, so
`->add('unirate_api', ['api_key' => getenv('UNIRATE_API_KEY')])` already
works on master; only the docs were missing.

- README.md: add row to commercial providers table (alphabetical
  position between `open_exchange_rates` and `webservicex`); bump
  provider counts 30 → 31 in tagline, providers section, and the
  Exchanger sibling-package line.
- doc/readme.md: add `unirate_api` / `api_key` row to the commercial
  config table.

UniRateAPI is a commercial REST API for FX (latest rates on the free
tier; historical and commodities on the paid tier — same shape as the
other paid providers in the table).

Author: rob-browncc

README.md

@@ -5,7 +5,7 @@
 [![Total Downloads](https://img.shields.io/packagist/dt/florianv/swap.svg?style=flat-square)](https://packagist.org/packages/florianv/swap)
 [![Version](http://img.shields.io/packagist/v/florianv/swap.svg?style=flat-square)](https://packagist.org/packages/florianv/swap)
 
-> _The easy-to-use PHP currency conversion library. Retrieve exchange rates from 30 providers, with caching and fallback. Maintained since 2014._
+> _The easy-to-use PHP currency conversion library. Retrieve exchange rates from 31 providers, with caching and fallback. Maintained since 2014._
 
 <table>
    <tr>
@@ -101,7 +101,7 @@ For amount conversion (including the [moneyphp/money](https://github.com/moneyph
 
 ## 📊 Providers
 
-Swap supports 30 exchange rate providers. Pass the **identifier** to `Builder::add()`.
+Swap supports 31 exchange rate providers. Pass the **identifier** to `Builder::add()`.
 
 ### Commercial providers (require an API key)
 
@@ -123,6 +123,7 @@ Swap supports 30 exchange rate providers. Pass the **identifier** to `Builder::a
 | Fixer (direct)                           | `fixer`                        | EUR (free), * (paid) | *     | Yes        |
 | 1Forge                                   | `forge`                        | *                    | *     | No         |
 | Open Exchange Rates                      | `open_exchange_rates`          | USD (free), * (paid) | *     | Yes        |
+| UniRateAPI                               | `unirate_api`                  | *                    | *     | Yes        |
 | WebserviceX                              | `webservicex`                  | *                    | *     | No         |
 | xChangeApi.com                           | `xchangeapi`                   | *                    | *     | Yes        |
 | Xignite                                  | `xignite`                      | *                    | *     | Yes        |
@@ -162,7 +163,7 @@ You can also add your own provider by implementing the `Exchanger\Contract\Excha
 The Swap ecosystem is a layered toolkit for currency conversion in PHP:
 
 - [**Swap**](https://github.com/florianv/swap). The easy-to-use, high-level API (this package).
-- [**Exchanger**](https://github.com/florianv/exchanger). Lower-level, more granular alternative; direct access to the 30 provider implementations and the `ExchangeRateService` interface.
+- [**Exchanger**](https://github.com/florianv/exchanger). Lower-level, more granular alternative; direct access to the 31 provider implementations and the `ExchangeRateService` interface.
 - [**Laravel Swap**](https://github.com/florianv/laravel-swap). Laravel application of Swap.
 - [**Symfony Swap**](https://github.com/florianv/symfony-swap). Symfony integration of Swap.
 

doc/readme.md

@@ -281,6 +281,7 @@ Commercial providers require an API key. The option name varies by provider. The
 | `fixer_apilayer`                 | `api_key`       |                       |
 | `forge`                          | `api_key`       |                       |
 | `open_exchange_rates`            | `app_id`        | `enterprise` (bool)   |
+| `unirate_api`                    | `api_key`       |                       |
 | `xchangeapi`                     | `api-key`       | (note the hyphen)     |
 | `xignite`                        | `token`         |                       |