Update README.md

[rumaniel]

Hi, Thank you for making this awesome repo.

But when I tried to install this package via Git URL, I faced an error so I tried to fix a minor docs update.

Fix unable to add package error on Unity Package Manager.

Problem

When trying to add git from the 'Install via Git URL', it faced an error:

[Package Manager Window] Cannot perform upm operation: Unable to add package [https://github.com/hk1ll3r/ReverseGeocoderForUnity]:
  Package name 'https://github.com/hk1ll3r/ReverseGeocoderForUnity' is invalid. [InvalidParameter].
[Package Manager Window] Error adding package: https://github.com/hk1ll3r/ReverseGeocoderForUnity.