Difficulty update

yup! a day after release. someone complained that the game was too easy so... here i come.

this update includes:

• finally giving the tutorial a thumbnail
• 3 new enemy types
• a slight adjustment to enemy spawn chances on the acid map.
• slight adjustment to the wave progression and special enemy spawning mechanics

explosive enemy (it only appears on the acid map)

electric enemy (appears randomly on the acid map, only after wave 15 on the city map.)

rocket enemy (same spawns as the electric one)

this is it for the update

Have fun! <3