Added i2c submodule

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "uart"]
 	path = uart
 	url = git@git.blackmark.me:avr/uart.git
+[submodule "i2c"]
+	path = i2c
+	url = git@git.blackmark.me:avr/i2c.git

README.md

@@ -65,7 +65,7 @@ The PWM mode of the hardware timer can also be used. Additionally, if PWM is nee
     - [ ] Compile-time check for correct hardware configuration
     - [ ] Software backend driver
 - I2C
-    - [ ] Separation of interface and backend-driver
+    - [x] Separation of interface and backend-driver
     - [ ] Support all hardware provided configuration options
     - [ ] Compile-time check for correct hardware configuration
     - [ ] Software backend driver