Warn about using rand() and the seed functions

Using `$faker` with the seed functionality `$faker->seed(1337)` and PHP function `rand()` break seed:

```php
<?php
require_once 'Faker-master/src/autoload.php';

$faker = Faker\Factory::create('en_AU');
$faker->seed(1337);

for ($i = 0; $i < 3; $i++) {
    print $faker->numberBetween(1, 10) . PHP_EOL;
    print $faker->realText(rand(10,20)) . PHP_EOL;
}
```
First run generate
>8
>I fell.
>8
>I did:.
>8
>Morcar,.

Second run generate 
>8
>I fell off.
>4
>Queen, and.
>10
>.

*ALWAYS* using $faker is the key to avoid this problem
```php
<?php
require_once 'Faker-master/src/autoload.php';

$faker = Faker\Factory::create('en_AU');
$faker->seed(1337);

for ($i = 0; $i < 3; $i++) {
    print $faker->numberBetween(1, 10) . PHP_EOL;
    //use faker instead of rand()
    print $faker->realText($faker->numberBetween(10,20)) . PHP_EOL;
}
```
First run
>8
Pigeon; 'but.
6
This time there.
6
The Mouse looked.

Second run 
>8
Pigeon; 'but.
6
This time there.
6
The Mouse looked.