Added links to registries

Signed-off-by: Sara Damiano <sdamiano@stroudcenter.org>

Author: SRGDamia1

README.md

@@ -1,3 +1,21 @@
 # GroPointModbus<!--! {#mainpage} -->
 
 A library to use an Arduino as a master to control and communicate via modbus with GroPoint soil moisture sensors. Depends on the EnviroDIY/SensorModbusMaster library.
+
+<!--! @tableofcontents -->
+
+<!--! @m_footernavigation -->
+
+<!--! @if GITHUB -->
+
+- [GroPointModbus](#gropointmodbus)
+  - [Library installation](#library-installation)
+
+<!--! @endif -->
+
+## Library installation
+
+This library is available through both the Arduino and PlatformIO library registries.
+[Here is the PlatformIO registry page.](https://registry.platformio.org/libraries/envirodiy/GroPointModbus)
+Use the Arduino IDE to find the library in that registry.
+The build and ingest logs for this library into the Arduino library registry are available [here](https://downloads.arduino.cc/libraries/logs/github.com/EnviroDIY/GroPointModbus/).