Add howto.md

howto.md

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+1. Declare functions in a header file using ifndef macros to prevent multiple includes:
+
+```c
+/* example.h file */
+#ifndef EXAMPLE_H
+#define EXAMPLE_H
+
+void example(void);
+
+#endif
+```
+
+Do this for each file. In this example `another-example.c`, `third-example.c`.
+
+2. Create implementation / definition of that function:
+
+```c
+/* example.c file */
+void example(void);
+{
+    /* do something */
+}
+```
+
+3. Compile to object file: `clang -c example.c another-example.c third-example.c`. You can also do `clang -c *.c`.
+
+To create archive (static library): `llvm-ar r outputname.a example.c another-example.c third-example.c`
+
+To create shared object (dynamic library): `ld.lld --shared -o outputname.so example.c another-example.c third-example.c`
+
+Use either with your main:
+
+Archive: `clang main.c outputname.a`
+
+---
+
+Shared object: `clang main.c outputname.so`
+
+On Linux also set correct LD_LIBRARY_PATH with `export LD_LIBRARY_PATH=/path/to/outputname.so`