Added documentation on how to add compatibility tests (#497)

* Added documentation on how to add compatibility tests

* Fixed grammar issues

compatibility/README.md

@@ -0,0 +1,49 @@
+# Bincode compatibility test
+
+Hello! We are working on releasing bincode 2, and we want to make sure that bincode 2 produces the same output as bincode 1.3.
+
+For this, we need your help. Please read on if you're using bincode in one of your projects.
+
+We have created a test project that can be used to test the compatibility between bincode 1 and 2. It encodes structs with a wide range of settings, checks if the outputs are the same, and then deserializes the struct and checks if the output is the same.
+
+## Adding a test case for your project
+
+To add a test case for your project, please follow the following steps:
+- [ ] Fork https://github.com/bincode-org/bincode
+- [ ] create a new file `compatibility/src/<name>.rs`.
+- [ ] Add a link to your project
+- [ ] Add `Licence: MIT OR Apache-2.0` if you agree to distribute your code under this license
+- [ ] Add a `mod <name>;` in the `lib.rs`. Make sure it's alphabetically ordered (check the ordering in your file system).
+- [ ] Add your structs.
+  - Adding references to libraries is not recommended. Libraries will not be implementing `bincode 2`'s encoding/decoding system.
+  - If you need references to libraries, consider adding a test case for that library, and then referencing that test.
+- [ ] Make sure structs derive the following traits:
+  - [ ] `serde::Serialize` and `serde::Deserialize`, like normal
+  - [ ] `bincode_2::Encode` and `bincode_2::Decode`, for the bincode 2 encode/decode mechanic
+  - [ ] Because the crate is renamed to `bincode_2`, you also need to add `#[bincode(crate = "bincode_2")]`
+  - [ ] `Debug, PartialEq`
+
+```rs
+#[derive(Serialize, Deserialize, bincode_2::Encode, bincode_2::Decode, Debug, PartialEq)]
+#[bincode(crate = "bincode_2")]
+pub struct YourStruct {
+}
+```
+
+- [ ] Use `rand` to be able to generate a random test case for your project.
+  - [ ] For strings there is a helper function in `crate`: `gen_string(rng: &mut impl Rng) -> String`
+- [ ] Add the following code:
+
+```rs
+#[test]
+pub fn test() {
+    let mut rng = rand::thread_rng();
+    for _ in 0..1000 {
+        crate::test_same(<your_rand_function>(&mut rng));
+    }
+}
+```
+
+For examples, see the existing cases in `compatibility/src/`.
+
+- [ ] Open a [pull request](https://github.com/bincode-org/bincode/pulls) with the title `Bincode 1 compatiblity: <name of your project>`