examples: rpmsg-proxy-app: add README

README file explains operation of this app and how to interact with
this demo. It also points to corresponding firmware of remote processor.

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>

examples/linux/rpmsg-proxy-app/README

@@ -0,0 +1,44 @@
+app: proxy_app
+
+Description:
+
+  This app demonstrates two functionality
+  1) Use of host processor's file system by remote processor
+  2) remote processor's standard IO redirection to host processor's standard IO
+
+  1) This app allows remote processor to use file system of host processor. Host
+  processor file system acts as proxy of remote file system. Remote processor
+  can use open, read, write, close calls to interact with files on host
+  processor.
+
+  File "remote.file" is available after app exits on host side that is created by
+  remote processor that contains string "This is a test string being written to
+  file.." written by remote firmware. This demonstrates remote firmware can
+  create and write files on host side.
+
+  2) This application also demonstrates redirection of standard IO.
+  Remote processor can use host processor's stdin and stdout via proxy service
+  that is implemented on host side. This is achieved with open-amp proxy
+  service implemented here:
+  https://github.com/OpenAMP/open-amp/blob/main/lib/proxy/rpmsg_retarget.c
+  Remote side firmware uses two types of output functions to print message on
+  console 1) xil_printf i.e. using same UART console as of APU and 2) Standard
+  "printf" function that is re-directed to standard output of Host. Both function
+  uses different ways to output messages, but using same console.
+
+  This is interactive demo:
+  1. When the remote application prompts you to Enter name, enter any string without space.
+  2. When the remote application prompts you to Enter age , enter any integer.
+  3. When the remote application prompts you to Enter value for pi, enter any floating
+     point number.
+  After this, remote application will print all the inputs entered by user on console
+  of host processor.
+
+  Remote firmware's standard IO are redirected to host processor's standard IO.
+  So, when remote uses "printf" and "scanf" functions actually host processor's
+  console is used for printing output and scanning inputs. Host communicates with
+  remote via rpmsg_char driver and Remote communicates to Host via redirected
+  Standard IO.
+
+Remote processor firmware: Xilinx ZynqMP cortex-r5 platform
+  https://github.com/OpenAMP/open-amp/blob/main/apps/examples/rpc_demo/rpc_demo.c