Adding README

Author: JandroMejia97

REAME.md

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+# ESP-32s Project with Arduino and PlatformIO
+
+This project consists of an ESP-32s that uses Arduino libraries to receive data (temperature, air/soil humidity, and light intensity) through UART and send it to an MQTT server on Ubidots. PlatformIO is used as the integrated development environment (IDE).
+
+## Before start...
+Before compiling the project, you must create a file named wifi_credentials.h with the following defined macros:
+
+```c++
+#define WIFI_SSID_SECRETS "<Network Name>"
+#define WIFI_PASSWORD_SECRETS "<Password>"
+#define UBIDOTS_TOKEN_SECRETS "<Ubidots Token>"
+#define DEVICE_LABEL_SECRETS "<Device name>"
+```
+
+| ⚠️ Make sure to replace the example values with your own data before compiling.
+
+## Customization
+
+Additionally, you can customize the project by overwriting the following constants:
+
+```c++
+#define DATA_UART <UART NUMBER>
+#define SERIAL_RX <PIN TO RECEIVE DATA>
+#define SERIAL_TX <PIN TO TRANSMIT DATA>
+#define BAUD_RATE <BAUD RATE TO TRANSMIT DATA>
+#define FREQUENCY_TO_PUBLISH_IN_M <FREQUENCY TO PUBLISH IN MINUTES>
+#define FREQUENCY_TO_PUBLISH_IN_MS (FREQUENCY_TO_PUBLISH_IN_M * 60 * 1000)
+```
+
+| ⚠️ Make sure that the slave device transmits at the same baud rate defined in the BAUD_RATE macro.