Add readme

Author: oxplot

Readme.md

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+**pdftilecut** lets you sub-divide a PDF page(s) into smaller pages so you
+can print them on small form printers. This operation is sometimes called
+*posterizing* (related to printing large posters on home printers) and
+*[tile cropping](http://www.imagemagick.org/Usage/crop/#crop_tile)*.
+
+# Installation
+
+Latest statically linked binaries [are available on releases
+page](https://github.com/oxplot/pdftilecut/releases).
+
+# Features
+
+* Statically compiled Go binaries with no OS dependencies.
+* Multi page PDF support. Each page can be of arbitrary size.
+* Each output page has appropriate PDF bleed and trim boxes set (so you
+  can give it to a printing house and get back correctly trimmed pages).
+* Print marks to assist with accurate trimming and arranging.
+* Small increase in output size.
+
+# Usage
+
+We want to print `mars.pdf` from the [beautiful NASA
+posters](https://www.jpl.nasa.gov/visions-of-the-future/)
+collection, which comes in 20x30 inch size, on A4 papers:
+
+```sh
+$ pdftilecut -tile-size A4 -in mars.pdf -out mars_a4.pdf
+```
+
+Below on the left is the orignal poster, and two of its tiles on the
+right:
+
+![Poster before and after](/img/example.png?raw=true "Poster before and
+after")
+
+The header contains the orignal page number and a tile reference for
+easier arranging:
+
+![Tile heading](/img/heading.png?raw=true "Tile heading")
+
+# Credits
+
+The amazing [QPDF library](https://github.com/qpdf/qpdf) is used to
+parse and write back modified PDF files. QPDF relies on
+[libz](https://www.zlib.net) and
+[libjpeg](https://github.com/libjpeg-turbo). Library sources are not
+included in this repo and must be downloaded separately.