Add a small README file for api/

Bug: none
Change-Id: Ied90a30724c3b3230af4aa4ffa8d4ee5b617f8a6
Notry: true
Reviewed-on: https://webrtc-review.googlesource.com/c/120605
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26510}

api/README.md

@@ -0,0 +1,24 @@
+# How to write code in the `api/` directory
+
+Mostly, just follow the regular [style guide](../style-guide.md), but:
+
+* Note that `api/` code is not exempt from the “`.h` and `.cc` files come in
+  pairs” rule, so if you declare something in `api/path/to/foo.h`, it should be
+  defined in `api/path/to/foo.cc`.
+* Headers in `api/` should, if possible, not `#include` headers outside `api/`.
+  It’s not always possible to avoid this, but be aware that it adds to a small
+  mountain of technical debt that we’re trying to shrink.
+* `.cc` files in `api/`, on the other hand, are free to `#include` headers
+  outside `api/`.
+
+That is, the preferred way for `api/` code to access non-`api/` code is to call
+it from a `.cc` file, so that users of our API headers won’t transitively
+`#include` non-public headers.
+
+For headers in `api/` that need to refer to non-public types, forward
+declarations are often a lesser evil than including non-public header files. The
+usual [rules](../style-guide.md#forward-declarations) still apply, though.
+
+`.cc` files in `api/` should preferably be kept reasonably small. If a
+substantial implementation is needed, consider putting it with our non-public
+code, and just call it from the `api/` `.cc` file.