README: add figure, quick-start example, and install instructions

- Add Barents Sea drifter figure from published docs
- Add mamba and pip install instructions
- Add quick-start code snippet covering plot, speed filter, gridtime, and animate
- Link to documentation gallery for further examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gauteh

README.md

@@ -5,10 +5,50 @@
 [![PyPI version](https://badge.fury.io/py/trajan.svg)](https://badge.fury.io/py/trajan)
 [![Anaconda-Server Badge](https://anaconda.org/conda-forge/trajan/badges/version.svg)](https://anaconda.org/conda-forge/trajan)
 
-TrajAn is a Python package with functionality to handle trajectory datasets following the CF-conventions on trajectories:
-http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#trajectory-data
+TrajAn is a Python package for working with trajectory datasets that follow the [CF-conventions for trajectories](http://cfconventions.org/Data/cf-conventions/cf-conventions-1.10/cf-conventions.html#trajectory-data). Trajectory datasets contain position time series from e.g. drifting buoys or output from Lagrangian models.
 
-Trajectory datasets contain position time series from e.g. drifting buoys, or output from lagrangian models.
+![Barents Sea drifter trajectories coloured by speed](https://opendrift.github.io/trajan/_images/sphx_glr_example_drifters_004.png)
 
-[Documentation and demonstration can be found here](https://opendrift.github.io/trajan/gallery)
+## Installation
+
+**conda / mamba** (recommended):
+```bash
+mamba install -c conda-forge trajan
+```
+
+**pip**:
+```bash
+pip install trajan
+```
+
+## Quick start
+
+TrajAn exposes a `.traj` accessor on xarray Datasets:
+
+```python
+import lzma
+import xarray as xr
+import trajan as ta
+
+# Open a CF-trajectory dataset (e.g. from drifting buoys)
+with lzma.open("barents.nc.xz") as f:
+    ds = xr.open_dataset(f)
+    ds.load()
+
+# Basic map plot — geographic projection is chosen automatically
+ds.traj.plot()
+
+# Calculate drifter speed, drop unreliable fixes, and plot coloured by speed
+ds = ds.traj.drop_where(ds.traj.time_to_next() < np.timedelta64(5, "m"))
+speed = ds.traj.speed()
+ds.traj.plot(color=speed)
+
+# Interpolate to a regular 1-hour time grid
+dh = ds.traj.gridtime("1h")
+
+# Animate the trajectories
+ds.traj.animate().show()
+```
+
+For more examples and the full API reference see the **[documentation](https://opendrift.github.io/trajan/gallery)**.