Fix missing references in README overview section

[Aymeric-e]

To resolve issue #80 "[Documentation] Missing references in the README overview".

The following items appear to be missing from the overview:

Higher or Lower
Path: Games/Higher_lower

Gamble
Path: Games/gambler

Memorise cards
Path: Games/memory_game

Encoding and decoding images
Path: Tools/Image encoding and decoding

Modified the README.md file to add the missing games and tools to the overview section of the file.