Update CUSTOMIZE.md for Newsletter support (alshedivat#2521)

In reference to alshedivat#2517 and
alshedivat#2517 (comment)

Author: asboyer

CUSTOMIZE.md

@@ -146,3 +146,9 @@ A variety of beautiful theme colors have been selected for you to choose from. T
 ## Adding social media information
 
 You can add your social media links by adding the specified information at the `Social integration` section in the [\_config.yml](_config.yml) file. This information will appear at the bottom of the `About` page.
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+## Adding a newsletter
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+You can add a newsletter subscription form by adding the specified information at the `newsletter` section in the [\_config.yml](_config.yml) file. To set up a newsletter, you can use a service like [Loops.so](https://loops.so/), which is the current supported solution. Once you have set up your newsletter, you can add the form [endpoint](https://loops.so/docs/forms/custom-form) to the `endpoint` field in the `newsletter` section of the [\_config.yml](_config.yml) file.
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+Depending on your specified footer behavior, the sign up form either will appear at the bottom of the `About` page and at the bottom of blogposts if `related_posts` are enabled, or in the footer at the bottom of each page.