Add diagrams

docs/README.md

@@ -256,6 +256,18 @@ struct mg_connection {
   By default, IO buffer allocation size `MG_IO_SIZE` is 2048: change it to 512
   to trim run-time per-connection memory consumption.
 
+## Architecture diagram
+
+In the Operating System environment, Mongoose uses BSD sockets API provided
+by the OS's TCP/IP stack:
+
+![](images/arch2.svg)
+
+In the embedded bare metal environment, Mongoose can utilise its own built-in
+stack with network drivers - i.e. it can run directly on top of the hardware:
+
+![](images/arch1.svg)
+
 ## Build options
 
 Mongoose source code ships in two files:
@@ -355,7 +367,6 @@ systems, follow the outline below:
    are relevant to your platform. Mongoose uses `bool` type, `MG_DIRSEP` define,
    and optionally other structures like `DIR *` depending on the functionality
    you have enabled - see previous section. Below is an example:
-
    ```c
    #include <stdbool.h>
    #include <stdarg.h>
@@ -363,11 +374,10 @@ you have enabled - see previous section. Below is an example:
    #define MG_DIRSEP '/'
    #define MG_INT64_FMT "%lld"
    ```
-You can also add
+   You can also add a `MG_ARCH` definition:
    ```c
    #define MG_ARCH MG_ARCH_CUSTOM
    ```
-To this file, instead of adding build flags.
 3. This step is optional, and only required if you intend to use a custom
    TCP/IP stack. To do that, you should:
   * Disable BSD socket API: in the `mongoose_custom.h`, add