update README.md

JIRA: CI-418

README.md

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+# phoenix-rtos-doc
+
+The full documentation can be found on <https://docs.phoenix-rtos.com/>.
+
+This repository contains all the necessary files needed to build
+Phoenix-RTOS Documentation using [https://github.com/sphinx-doc/sphinx](sphinx).
+
+It can be done using the following command:
+
+```console
+make clean html
+```
+
+The artifacts will be available in `_build` directory.