Update README

Author: rschroll

README

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+rmrl: reMarkable Rendering Library
+===================================
+rmrl is a Python library for rendering reMarkable documents to PDF files.
+It takes the original PDF document and the files describing your annotations,
+combining them to produce a document close to what reMarkable itself would
+output.
+
+Demo
+----
+The same notebook was rendered to a PDF via the reMarkable app and rmrl.
+The resultant PDF files were converted to PNGs with ImageMagick at 300
+dpi.
+
+ reMarkable output | rmrl output
+:-----------------:|:-----------:
+[![reMarkable](demo/app.png)](demo/app.png) | [![rmrl](demo/rmrl.png)](demo/rmrl.png)
+
+The biggest differences are the lack of texture in the pencils and paintbrush,
+which we hope to address in the future.  The highlight color is also different,
+but we feel the default color is too subtle.
+
+Installation
+------------
+rmrl requires Python 3.7 or later.  If that's installed, the easiest installation
+is to do a
+```bash
+pip install rmrl
+```
+Alternatively, you should be able to clone this repository and run
+```bash
+pip install -e .
+```
+
+Usage
+-----
+The main interface to rmrl is through a single function:
+```python
+from rmrl import render
+
+output = render(source)
+```
+`source` may be:
+- The filename of a zip file containing the document.
+- The filename of any (root-level) file from an unpacked document.
+- Any object that provides `open()` and `exists()` methods.  See
+  `rmrl/sources.py` for more details on this API.
+
+The output is a filestream with the contents of the PDF file.
+
+The `render` function takes the following keyword arguments:
+- `progress_cb`: A callback function to be called periodically during the
+  rendering process.  It will be called with a single argument, a number
+  from 0 to 100 indicating the progress.  This function can abort the
+  process by raising an exception.
+
+Command-line Usage
+------------------
+rmrl may be called as a command-line tool.  Once it has been installed, run
+```bash
+python -m rmrl filename
+```
+to convert `filename` to an annotated PDF.  The default output is to stdout.
+Use
+```bash
+python -m rmrl -h
+```
+to see all of the options.
+
+Templates
+---------
+rmrl can use the reMarkable templates as a background when rendering notebooks.
+We cannot ship copies of these templates.  You may be allowed to copy them from
+your own reMarkable device on to your computer for personal use.  If this is
+legal in your jurisdiction, you may connect your device to your computer by the
+USB cable and run
+```bash
+python -m rmrl.load_templates
+```
+This will copy these templates to `~/.local/share/rmrl/templates` (assuming
+default XDG settings).
+
+History
+-------
+rmrl derives from the [reMarkable Connection Utility](http://www.davisr.me/projects/rcu/),
+by Davis Remmel.  RCU is a full-featured GUI for managing all aspects of a
+reMarkable device.  Do check it out if you are looking for a stand-alone
+solution for getting documents on and off of your device.
+
+RCU was chosen as a base for rmrl due to its high-quality rendering.  The
+following are the major changes:
+- rmrl is designed as a library, for incorporation into other programs.  RCU
+  is designed as a stand-alone program.
+- rmrl uses the pure-Python [ReportLab Toolkit](https://www.reportlab.com/dev/opensource/rl-toolkit/)
+  for rendering PDF files.  RCU uses the Qt framework, which is a significantly
+  heavier installation.
+- rmrl only supports vector output, while RCU offers both raster and vector
+  rendering.
+- RCU supports PDF layers (Optional Content Groups).  At this point, rmrl does
+  not.
+- RCU can add PDF annotations corresponding to highlights.  At this point, rmrl
+  does not.
+
+Trademarks
+----------
+reMarkable(R) is a registered tradebark of reMarkable AS. rmrl is not
+affiliated with, or endorsed by, reMarkable AS. The use of “reMarkable”
+in this work refers to the company’s e-paper tablet product(s).
+
+Copyright
+---------
+Copyright (C) 2020  Davis Remmel
+
+Copyright 2021 Robert Schroll
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.