Adapted the README.md to the new package layout.

README.md

 # toarstats
 
-This repository contains a collection of statistics that can be
-calculated on **hourly** data. The statistics in the `ozone_metrics.py`
-file are specific to ozone data. The statistics in the `stats.py` file
-can be calculated for other variables as well.
+This repository contains a collection of statistical tools for the
+analysis of time series data. It is split into two subpackages:
+- metrics: collection of statistics and metrics to calculate on
+           **hourly** time series data (some specific to ozone
+           measurements)
+- trends: calculate quantile regression on time series data
 
 ## Installation
 
@@ -14,15 +16,20 @@ python3 -m pip install toarstats-<version>-py3-none-any.whl
 ```
 It is advised to set up a virtual environment beforehand.
 
-## Usage
+## metrics
+
+This subpackage contains a collection of statistics that can be
+calculated on **hourly** data. The statistics in the `ozone_metrics.py`
+file are specific to ozone data. The statistics in the `stats.py` file
+can be calculated for other variables as well.
 
 ### Import
 
 To use the package import `calculate_statistics` with:
 ```
-from toarstats import calculate_statistics # or
-from toarstats import * # or
-import toarstats
+from toarstats.metrics import calculate_statistics # or
+from toarstats.metrics import * # or
+import toarstats.metrics
 ```
 
 ### Interface