Adds docs for watcher-c

watcher-c/readme.md

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+# Using the watcher library from C
+
+The watcher can also be used from C by including the `watcher-c.h` file on compilation and in your project.
+
+You can cross compile the library via the `watcher-c/cross-compile.sh` script or download the precompiled library from the releases.
+
+### Compiling the library from source
+
+If you want to compile the library yourself you can follow the following example:
+
+First check out the `next` branch, compile a shared `libwatcher-c.so` library with `gcc` and install
+it in the `/usr/local/lib` directory:
+
+```bash
+curl -L https://github.com/e-dant/watcher/archive/refs/heads/next.tar.gz | tar xz
+cd watcher-next/watcher-c
+gcc -o libwatcher-c.so ./src/watcher-c.cpp -I ./include -I ../include -std=c++17 -O3 -Wall -Wextra -fPIC -shared
+cp libwatcher-c.so /usr/local/lib/libwatcher-c.so
+ldconfig
+```
+
+### Using the library in your C code
+
+After installing the library, copy the `watcher-c.h` file to your project and include it in your C code:
+
+```c
+#include "watcher-c.h"
+```
+
+You will then be able to use the `wtr_watcher_open` and `wtr_watcher_close` functions by linking against the library (`-lwatcher-c`).
+
+`wtr_watcher_close` will always return `false` if the watcher is closed before the first `@live` event has been sent.
+
+Note that the watcher depends on the c++ standard library.