Update INSTALL_FROM_GITHUB.md with account/key info (openframeworks#7606

)

INSTALL_FROM_GITHUB.md

@@ -14,6 +14,13 @@ git clone --recursive git@github.com:openframeworks/openFrameworks.git --depth 1
 
 _Before continuing make sure your new openFrameworks path **has no spaces**. Many of the shell scripts below will fail on paths that include spaces._
 
+If you get this error:
+```
+git@github.com: Permission denied (publickey).
+fatal: Could not read from remote repository.
+```
+it means you need a form of authentication into github. This implies you have a github account, properly configured for your machine. There are different ways to get there, but the simplest path is by going into your github account  Settings, navigate to the GPG keys, and add the public key for the computer you are cloning into.
+
 ### Download dependencies
 
 As the external dependencies are not found in the repository, you need to download them. To make things simpler, use the bash script called `download_libs.sh` which can be found in the `scripts` folder. In this folder, there are several subfolders, one for each platform. Assuming you are, for example, using OSX, you need to run `/bin/bash scripts/osx/download_libs.sh`.