Add X52 MFD pages document

This document describes the API to update pages on the MFD.

docs/x52_mfd_pages.md

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+X52 MFD pages
+=============
+
+The Saitek SDK for Windows allows the programmer to create multiple
+"pages" on the MFD display which can be toggled using the PgUp and PgDn
+controls below the MFD. In addition to the pages, the software also
+allows the programmer to write more than 16 characters to each line and
+will automatically scroll those lines.
+
+# Features
+
+* Create multiple pages, each page containing up to 3 lines of text and
+each line containing upto 256 characters.
+* Automatically shift the text by 1 character every 250 ms, iff the text
+exceeds 16 characters.
+* Support callbacks on a per-page basis for the Up, Down and Select
+buttons, to allow the programmer to create custom handlers.
+* Support a summary page listing all the active pages.
+
+# API
+
+    x52_mfd_create_page(uint8_t *page_id,
+                        char *description,
+                        x52_page_callback *handler)
+
+    x52_mfd_write_line(uint8_t page_id,
+                       uint8_t line,
+                       char *text,
+                       uint8_t length)
+
+    x52_mfd_update_page(uint8_t page_id,
+                        uint8_t activate)
+
+    x52_mfd_delete_page(uint8_t page_id)
+
+    typedef void (*x52_page_callback)(uint8_t page_id,
+                                      uint8_t event,
+                                      uint8_t state)
+
+## Steps
+
+1. Create a page
+2. Write the individual lines of text to the page
+3. Update the page with activate parameter set to 1
+4. (Optional) Deactivate the page with activate parameter set to 0
+5. Delete the page
+
+## Callback
+
+The callback API is a function that takes three arguments as follows:
+
+1. A page ID (8 bits) which identifies the current page. This allows
+   a single callback handler to handle multiple pages.
+2. An 8-bit event identifier. Currently, this is defined to be one of:
+   * X52\_MFD\_EVENT\_UP 
+   * X52\_MFD\_EVENT\_DN
+   * X52\_MFD\_EVENT\_SEL
+3. An 8-bit state identifier. Currently, this is defined to be one of:
+   * X52\_MFD\_BUTTON\_STATE\_UP
+   * X52\_MFD\_BUTTON\_STATE\_DN
+
+