Add I2C slave library

- Add I2C slave library for general use.

drivers/avr/i2c_slave.h

@@ -0,0 +1,19 @@
+/* Library made by: g4lvanix
+ * Github repository: https://github.com/g4lvanix/I2C-slave-lib
+
+ Info: Inititate the library by giving the required address.
+       Read or write to the necessary buffer according to the opperation.
+ */
+
+#ifndef I2C_SLAVE_H
+#define I2C_SLAVE_H
+
+volatile uint8_t buffer_address;
+volatile uint8_t txbuffer[0xFF];
+volatile uint8_t rxbuffer[0xFF];
+
+void i2c_init(uint8_t address);
+void i2c_stop(void);
+ISR(TWI_vect);
+
+#endif // I2C_SLAVE_H