doc: add README

README.md

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+# Evalexpr
+
+This was a small experiment in writing a precedence climbing parser, using C.
+I also have my reference parsing implementation, using recursive descent, to
+compare the parsing results.
+
+This is mostly based on the explanation found at this address:
+https://www.engr.mun.ca/~theo/Misc/exp_parsing.htm (use the Wayback Machine to
+read it).
+
+## How to build
+
+Simply launch the following command
+
+```sh
+42sh$ make
+```
+
+If you want to build an `evalexpr` command using recursive descent instead, use:
+
+```sh
+42sh$ make USE_CLIMBING=0
+```
+
+Don't forget to use `make clean` when alternating between both.
+
+
+## How to use
+
+Simply launch the binary, and write an expression on its standard input. The
+binary can parse exactly one expression per line, and reports parsing errors if
+they happen.
+
+Example use:
+
+```none
+42sh$ ./evalexpr
+1 + 2 * 3 - 3!
+1
+```