Update README.md (#8)

* Update README.md

Added usage section

* Update README.md

Typo fix and Python syntax

README.md

@@ -1,6 +1,6 @@
 # libcosimpy 
 
-Python wrapper for the [libcosim](https://github.com/open-simulation-platform/libcosim/tree/master/src/cosim) library. The wrapper uses the [libcosimc](https://github.com/open-simulation-platform/libcosimc) C wrapper and the python [ctypes](https://docs.python.org/3/library/ctypes.html) library to make OSP accessible to python developers. 
+Python wrapper for the [libcosim](https://github.com/open-simulation-platform/libcosim/tree/master/src/cosim) library. The wrapper uses the [libcosimc](https://github.com/open-simulation-platform/libcosimc) C wrapper and the [ctypes](https://docs.python.org/3/library/ctypes.html) library to make OSP accessible to Python developers.   
 
 # Getting Started
 
@@ -12,8 +12,143 @@ To install from the source, run the following command at the root directory of t
 ```bash
 pip install .
 ```
-`libcosimpy` requires [ctypes](https://docs.python.org/3/library/ctypes.html) to call `libcosimc` functions. `ctypes` is included with python and does not have to be installed.
+`libcosimpy` requires [ctypes](https://docs.python.org/3/library/ctypes.html) to call `libcosimc` functions. `ctypes` is included with Python and does not have to be installed.
 
-## Tests
+# Usage
 
-Tests can be run using the `pytest` command in the terminal. `libcosimc` log level for all tests can be set in the `./tests/conftest.py` file.
+## Create execution
+
+Import `CosimExecution` from `libcosimpy`
+
+```python
+from libcosimpy.CosimExecution import CosimExecution
+```
+
+#### Empty execution object
+
+```python
+execution = CosimExecution.from_step_size(step_size=1e3)
+```
+
+With a 0.01s fixed time step
+
+#### From OSP config
+
+```python
+execution = CosimExecution.from_osp_config_file(osp_path=f'[PATH_TO_OSP_DIRECTORY]')
+```
+
+#### From SSP config
+
+```python
+execution = CosimExecution.from_ssp_file(ssp_path=f'[PATH_TO_SSP_DIRECTORY]')
+```
+
+## Add slave
+
+FMUs can be added manually to execution. OSP and SSP config executions will import all required slaves automatically and this step is not required  
+
+Import `CosimLocalSlave` from `libcosimpy`
+
+```python
+from libcosimpy.CosimSlave import CosimLocalSlave
+```
+
+Add slave to existing execution 
+
+```python
+local_slave = CosimLocalSlave(fmu_path=f'[PATH_WITH_FILENAME_TO_FMU]', instance_name='[SOME_UNIQUE_NAME]')
+slave_index = execution.add_local_slave(local_slave=local_slave)
+```
+
+Slave index is used for future referencing to the model
+
+## Run simulation
+
+Simulations can either be run continiously for a duration
+
+```python
+execution.simulate_until(target_time=10e9)
+```
+
+To simulate for 10s
+
+Or stepped manually
+
+```python
+execution.step()
+```
+
+With option for stepping multiple steps at once
+
+```python
+execution.step(step_count=10)
+```
+
+## Finding slave and variable indices
+
+List of slave indices and corresponding indices can be fetched from execution
+
+```python
+slave_infos = list(execution.slave_infos())
+```
+
+List of model variables and corresponding indices can be fetched
+
+```python
+variables = execution.slave_variables(slave_index=slave_index)
+```
+
+The indices can also be found by unzipping the FMU-file and inspecting the `modelDescription.xml` file 
+
+## Retrieving values from simulation
+
+Import `CosimObserver` from `libcosimpy`
+
+```python
+from libcosimpy.CosimObserver import CosimObserver
+```
+
+Observers can be used to retrieve values as Python list
+
+```python
+observer = CosimObserver.create_last_value()
+execution.add_observer(observer=observer)
+
+# Run simulation
+...
+# Retrieve floating point values
+values = observer.last_real_values(slave_index=[SLAVE_INDEX], # Model to monitor (integer)
+                                   variable_references=[VALUE_REFERENCE(s)]) # List of indices to monitor (integer)
+```
+
+Time series and file export observers are also supported 
+
+## Overriding values in simulation
+
+Import `CosimManipulator` from `libcosimpy`
+
+```python
+from libcosimpy.CosimManipulator import CosimManipulator
+```
+
+Manipulators are used to override values
+
+```python
+manipulator = CosimManipulator.create_override()
+execution.add_manipulator(manipulator=manipulator)
+
+# Run simulation
+...
+# Override floating point values
+manipulator.slave_real_values(slave_index=[SLAVE_INDEX], # Model to monitor (integer) 
+                              variable_references=[VALUE_REFERENCE(s)], # Index or list of indices to manipulate (integer)
+                              values=[SOME_OVERRIDE_VALUE(s)]) # Floating point values used for override. Equal length to variable references
+execution.step()
+```
+
+Scenario manipulators are also supported
+
+# Tests
+
+Tests can be run using the `pytest` command in the terminal. `libcosimc` log level for all tests can be set in the `./tests/conftest.py` file.