Add README and LICENSE.

LICENSE

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+Copyright (c) 2020 Érico Nogueira
+
+Permission to use, copy, modify, and distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

README.md

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+# purr-c
+
+This is a pure C client for the
+[PurritoBin](https://github.com/PurritoBin/PurritoBin) pastebin server, and uses
+PurritoBin's author's instance in <https://bsd.ac> by default.
+
+It was written as an exercise in learning network and crypto libraries, and uses
+[BearSSL](https://www.bearssl.org/) for symmetric encryption (supported by
+PurritoBin's online interface as well as its [suggested
+clients](https://github.com/PurritoBin/PurritoBin/tree/master/clients)) and SSL
+interaction with the server. For now, it also depends on
+[s6-networking](http://skarnet.org/software/s6-networking/)'s `sbearssl` library
+for certificate parsing.
+[libbaseencode](https://github.com/paolostivanin/libbaseencode) has been
+vendored in (can be found in `external/libbaseencode`), but has also gone
+through some changes, namely removing null-byte checking from the base64
+encoding function and adding an output length parameter to the base64 decoding
+function.
+
+The code has a few instances of `// TODO: remove hack` comments and the like,
+which I hope to get to someday.
+
+## Usage
+
+Usage information can be viewed with `purr -h`.
+
+## Build dependencies
+
+The build dependencies are the ones listed above: BearSSL and s6-networking, as
+well as GNU Make and a C compiler. This currently depends on Linux's
+`getrandom(2)` function, but I plan on adding at least an `arc4random_buf(3)`
+implementation.
+
+## Acknowledgements
+
+- Thomas Pornin for BearSSL
+- Laurent Bercot for s6-networking
+- epsilon-0 for PurritoBin (and the valuable help while I was testing this
+   program)
+- paolostivanin for libbaseencode