doc: Add install instructions to README.md

Author: johan-bjareholt

README.md

@@ -9,3 +9,13 @@ Client library for ActivityWatch in Python.
 
 Please see the documentation for usage and examples.
 
+## How to install
+
+To install the latest git version directly from github without cloning, run
+`pip install git+https://github.com/ActivityWatch/aw-client.git`
+
+To install from a cloned version, cd into the directory and run
+`poetry install` to install inside an virtualenv. If you want to install it
+system-wide it can be installed with `pip install .`, but that has the issue
+that it might not get the exact version of the dependencies due to not reading
+the poetry.lock file.