Install instructions: Add package from git URL doesn't work if git is not installed on the system

[bjornicus]

Following the installation instructions, I tried to add the package by git url, but it didn't work, and it isn't obvious right away why, since the error shows up in the unity console and not in the package manager window:

https://github.com/AccelByte/accelbyte-unity-sdk.git#15.15.0.
Unable to add package [https://github.com/AccelByte/accelbyte-unity-sdk.git#15.15.0]:
  No 'git' executable was found. Please install Git on your system then restart Unity and Unity Hub
UnityEditor.EditorApplication:Internal_CallUpdateFunctions ()

Since default Unity version control is not git, it's probably not uncommon for git not to be installed on the machine. It would be worth having a note in the install steps to make sure git is installed, and/or how to download the package tar and add it from tarball.