feat: add readme

README.txt

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+tk_sfl
+Tilo Klarenbeek single file libraries
+
+---
+
+Probably don't use these, if you really want to:
+
+
+*** tk_memory.h ***
+```
+#include "tk_memory.h"
+#define TK_MEMORY_IMPLEMENTATION
+
+int main() {
+    void* ptr = tk_malloc(1024);
+    tk_free(ptr);
+
+    tk_check_memory_leaks();
+}
+
+```
+
+Define DEBUG to initalize memory with OXFE, to detect uninitialized memory.
+Define DEBUG AND TK_MEMORY_TRACK to catch memory leaks when calling tk_check_memory_leaks().