add license and readme

LICENSE

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+Copyright (c) 2021 Thomas Lindner <tom@dl6tom.de>
+
+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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+A toy language compiler.
+
+# Build dependencies
+ - ANTLR4
+ - Meson
+ - Ninja
+ - C++ compiler
+
+# Runtime dependencies
+ - ANTLR4 C++ runtime
+ - QBE
+ - Assembler (the one in the C compiler will do)
+
+# Build steps
+```
+meson setup builddir
+ninja -C builddir
+```
+The `xc` binary is in `builddir/bootstrap/xc`. Optionally install it with
+```
+ninja -C builddir install
+```
+
+# Usage
+Use `xc` to compile xlang to QBE intermediate langage
+```
+xc -o foo.ssa foo.x
+```
+Use `qbe` to optimize and emit assembly
+```
+qbe -o foo.S foo.ssa
+```
+Assemble and link the binary (easiest to let the C compiler figure it out)
+```
+cc -o foo foo.S
+```