Create CODE_STYLE.md

Related to https://github.com/teloxide/teloxide/issues/55.

CODE_STYLE.md

@@ -0,0 +1,26 @@
+# Code style
+This is a description of a coding style that every contributor must follow. Please, read the whole document before you start pushing code.
+
+## Generics
+Generics are always written with `where`.
+
+Bad:
+
+```rust
+    pub fn new<N: Into<String>,
+               T: Into<String>,
+               P: Into<InputFile>,
+               E: Into<String>>
+    (user_id: i32, name: N, title: T, png_sticker: P, emojis: E) -> Self { ... }
+```
+
+Good:
+
+```rust
+    pub fn new<N, T, P, E>(user_id: i32, name: N, title: T, png_sticker: P, emojis: E) -> Self
+    where
+        N: Into<String>,
+        T: Into<String>,
+        P: Into<InputFile>,
+        E: Into<String> { ... }
+```