Added a small doc page for orcasong_plag

docs/orcasong_plag.rst

+OrcaSong Plag
+=============
+
+OrcaSong Plag is an alternative to orcasong, with (hopefully) more
+accessible features.
+It has a slightly reduced functionality (no plots), but apart from that
+does the same job as orcasong.
+
+Basic Use
+---------
+
+Import the main class, the FileBinner (see
+:py:class:`orcasong_plag.core.FileBinner`),
+like this:
+
+.. code-block:: python
+
+    from orcasong_plag.core import FileBinner
+
+The FileBinner allows to make nd histograms ("images") from calibrated and
+h5-converted root files.
+To do this, you can pass a list defining the binning. E.g., the following would
+set up the file binner to generate zt data:
+
+.. code-block:: python
+
+    bin_edges_list = [
+        ["pos_z", np.linspace(0, 10, 11)],
+        ["time", np.linspace(-50, 550, 101)],
+    ]
+
+    fb = FileBinner(bin_edges_list)
+
+Calling the object like this will show you the binning:
+
+.. code-block:: python
+
+    fb
+    >>> <FileBinner: ('pos_z', 'time') (10, 100)>
+
+As you can see, the FileBinner will produce zt data, with 10 and 100 bins,
+respectively.
+Convert a file like this:
+
+.. code-block:: python
+
+    fb.run(infile, outfile)
+
+Or event this for multiple files, which will all be saved in the given folder:
+
+.. code-block:: python
+
+    fb.run_multi(infiles, outfolder)
+
+Adding mc_info
+--------------
+
+To add info from the mc_tracks (or from wherever), you can define some
+function `my_mcinfo_extractor` which takes as an input a km3pipe blob,
+and outputs a dict mapping str to float.
+
+This will be saved as a numpy structured array "y" in the output file, with
+the str being the dtype names. Set up like follows:
+
+.. code-block:: python
+
+    fb = FileBinner(bin_edges_list, mc_info_extr=my_mcinfo_extractor)
+