Add an API-CONVENTIONS.md document.

We're starting to get quite a lot of these ALL-CAPS.md documents.
There's been enough questions around how to properly use types like
EVP_MD_CTX that we probably should write down some of these common
rules.

Change-Id: I125f4e82efb168a071b54ff76c5af34c42ff4800
Reviewed-on: https://boringssl-review.googlesource.com/9115
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/README.md b/README.md
index 4caedb7..b28e721 100644
--- a/README.md
+++ b/README.md
@@ -26,6 +26,7 @@
   * [PORTING.md](/PORTING.md): how to port OpenSSL-using code to BoringSSL.
   * [BUILDING.md](/BUILDING.md): how to build BoringSSL
   * [INCORPORATING.md](/INCORPORATING.md): how to incorporate BoringSSL into a project.
+  * [API-CONVENTIONS.md](/API-CONVENTIONS.md): general API conventions for BoringSSL consumers and developers.
   * [STYLE.md](/STYLE.md): rules and guidelines for coding style.
   * include/openssl: public headers with API documentation in comments. Also [available online](https://commondatastorage.googleapis.com/chromium-boringssl-docs/headers.html).
   * [FUZZING.md](/FUZZING.md): information about fuzzing BoringSSL.