Add a README.

Change-Id: I77b1d0efa6cddfcb162be693d53276822780540f
Reviewed-on: https://boringssl-review.googlesource.com/5790
Reviewed-by: Adam Langley <agl@google.com>

README.md

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+# BoringSSL
+
+BoringSSL is a fork of OpenSSL that is designed to meet Google's needs.
+
+Although BoringSSL is an open source project, it is not intended for general
+use, as OpenSSL is. We don't recommend that third parties depend upon it. Doing
+so is likely to be frustrating because there are no guarantees of API or ABI
+stability.
+
+Programs ship their own copies of BoringSSL when they use it and we update
+everything as needed when deciding to make API changes. This allows us to
+mostly avoid compromises in the name of compatibility. It works for us, but it
+may not work for you.
+
+BoringSSL arose because Google used OpenSSL for many years in various ways and,
+over time, built up a large number of patches that were maintained while
+tracking upstream OpenSSL. As Google's product portfolio became more complex,
+more copies of OpenSSL sprung up and the effort involved in maintaining all
+these patches in multiple places was growing steadily.
+
+Currently BoringSSL is the SSL library in Chrome/Chromium, Android (but it's
+not part of the NDK) and a number of other apps/programs.
+
+There are other files in this directory which might be helpful:
+
+  * [PORTING.md](PORTING.md): how to port OpenSSL-using code to BoringSSL.
+  * [BUILDING.md](BUILDING.md): how to build BoringSSL
+  * [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).