core: adjust README (#1896)

Signed-off-by: David Korczynski <david@adalogics.com>

src/README.md

@@ -1 +1,30 @@
-# Fuzz Introspector core
+# Fuzz Introspector Python Package
+
+Library for analysing fuzzer-relevant components of source code.
+
+Example:
+
+```python
+from fuzz_introspector.frontends import core
+
+SAMPLE="""void parse(const char *data, size_t size) {
+  return;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  parse((const char*)data, size)
+}
+"""
+
+source = core.analyse_source_file(SAMPLE, 'c')
+func = source.get_function_node('LLVMFuzzerTestOneInput')
+if func:
+    print(func.callsites())
+```
+
+Running the above:
+
+```sh
+$ python3 example.py
+[('parse', (122, 127))]
+```