add readme for libtock-core

core/README.md

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+# libtock-core
+
+Core crate of `libtock-rs`. It contains the architecture specific code of `libtock-rs`. In particular:
+
+ * the entry point
+ * `panic` and `alloc_error` handlers
+ * the syscalls
+ * the allocator (optional)
+
+It has three important feature flags
+
+ * `alloc` - allow for heap. Enables a linked list allocator.
+ * `custom_panic_handler` - disable the default panic  handler and allow definition of a custom one using `#[panic_handler]`
+ * `custom_alloc_error_handler` - disable the default alloc error handler and allow definition of a custom one using `#[alloc_error_handler]`
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0
+  ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license
+  ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+
+The contribution guidelines are identical to those of `libtock-rs` and can be found here: [contribution guidelines](../CONTRIBUTING.md)