You should really follow official best practices and create your own module and add that custom rendering there based on these instructions:
Then add that module to your site.
However there is quicker solution to be used only for development purposes IMHO.
Just add new Available Renderings item based on /sitecore/templates/Foundation/Experience Accelerator/Presentation/Available Renderings/Available Renderings template. Name it "Custom" or something similar and in Renderings field, add your desired rendering:
You will then see it in Toolbox:
Mind that the section name is taken from where the custom rendering sits not from your item created above.
Use this option only when there are no rendering variants or data sources under site Data folder needed for your custom rendering!
As you can see on picture, rendering is not usable as Rendering Variants are missing...
You can duplicate this item between sites or create some Powershell script to create it for you but the best automation is to use first option ;)