README updates. (#4)

* README updates.

Author: JonathanHenson

README.md

@@ -2,6 +2,100 @@
 
 C++ wrapper around the aws-c-* libraries. Provides Cross-Platform Transport Protocols and SSL/TLS implementations for C++.
 
+### Currently Included:
+
+* aws-c-common: Cross-platform primitives and data structures.
+* aws-c-io: Cross-platform event-loops, non-blocking I/O, and TLS implementations.
+* aws-c-mqtt: MQTT client.
+
+More protocols and utilities are coming soon, so stay tuned.
+
+## Building
+
+The C99 libraries are already included for your convenience when you specify the `-DBUILD_DEPS=ON` CMake argument.
+
+If you want to manage these dependencies manually (e.g. you're using them in other projects), simply specify 
+`-DCMAKE_INSTALL_PREFIX` to point to the directory where you have them installed.
+
+## Dependencies?
+
+### Windows and Apple
+There are no non-OS dependencies that AWS does not own, maintain, and ship.
+
+### Unix
+The most likely answer is: none that you should care about. 
+
+We do depend on an openssl compatible libcrypto implementation being in your path if you are on a
+unix system. Note: we do not actually use libssl. The most likely scenario is that libcrypto is already installed on your system. However, since
+many linux distributions these days do not provide a 32-bit libcrypto package, if you're trying to perform a 32-bit build you will 
+likely need to build and install libcrypto from source.
+
+## Common Usage
+
+To do anything with IO, you'll need to create a few objects that will be used by the rest of the library.
+
+For example:
+
+````
+    Aws::Crt::LoadErrorStrings();
+````
+
+Will load error strings for debugging purposes. Since the C libraries use error codes, this will allow you to print the corresponding
+error string for each error code.
+
+````
+    Aws::Crt::ApiHandle apiHandle;
+````
+This performs one-time static initialization of the library. You'll need it to do anything, so don't forget to create one.
+
+````
+    Aws::Crt::Io::EventLoopGroup eventLoopGroup(<number of threads you want>);
+````
+To use any of our APIs that perform IO you'll need at least one event-loop. An event-loop group is a collection of event-loops that
+protocol implementations will load balance across. If you won't have very many connections (say, more than 100 or so), then you
+most likely only want 1 thread. In this case, you want to pass a single instance of this to every client or server implementation of a protocol
+you use in your application. In some advanced use cases, you may want to reserve a thread for different types of IO tasks. In that case, you can have an
+instance of this class for each reservation.
+
+````
+     Aws::Crt::Io::TlsContextOptions tlsCtxOptions =
+        Aws::Crt::Io::TlsContextOptions::InitClientWithMtls(certificatePath.c_str(), keyPath.c_str());
+    /*
+     * If we have a custom CA, set that up here.
+     */
+    if (!caFile.empty())
+    {
+        tlsCtxOptions.OverrideDefaultTrustStore(nullptr, caFile.c_str());
+    }
+
+    uint16_t port = 8883;
+    if (Io::TlsContextOptions::IsAlpnSupported())
+    {
+        /*
+        * Use ALPN to negotiate the mqtt protocol on a normal
+        * TLS port if possible.
+        */
+        tlsCtxOptions.SetAlpnList("x-amzn-mqtt-ca");
+        port = 443;
+    }
+
+    Aws::Crt::Io::TlsContext tlsCtx(tlsCtxOptions, Io::TlsMode::CLIENT);
+````
+
+If you plan on using TLS, you will need a TlsContext. These are NOT CHEAP, so use as few as possible to perform your task.
+If you're in client mode and not doing anything fancy (e.g. mutual TLS), then you can likely get away with using a single
+instance for the entire application.
+
+````
+Aws::Crt::Io::ClientBootstrap bootstrap(eventLoopGroup);
+````
+
+Lastly, you will need a client or server bootstrap to use a client or server protocol implementation. Since everything is
+non-blocking and event driven, this handles most of the "callback hell" inherent in the design. Assuming you aren't partitioning
+threads for particular use-cases, you can have a single instance of this that you pass to multiple clients.
+
+For a working Mqtt example, see samples/mqtt_pub_sub/main.cpp.
+
 ## License
 
 This library is licensed under the Apache 2.0 License.