docs(general_notes): Add general notes to main page of document

Author: donghengqaz

Diff for: docs/_static/general-notes.gif

@@ -0,0 +1,25 @@
+*************
+General Notes
+*************
+
+Adding this content here is to improve the user's development efficiency and avoid stepping into known problems.
+
+1. Bootloader
+^^^^^^^^^^^^^
+
+V3.1 updated the bootloader to initialize SPI flash I/O mode and clock. So if you are using the V3.0 bootloader,
+and now upgrade to the new SDK, please disable the following configuration in the menuconfig:
+    
+    - "Bootloader config  ---> [ ] Bootloader init SPI flash"
+ 
+2. Sniffer or Smartconfig
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+We moved some functions from IRAM to flash, including `malloc` and `free` fucntions, to save more memory.
+In this case, please do not read/write/erase flash during sniffer/promiscuous mode.
+You need to disable the sniffer/promiscuous mode at first, then read/write/erase flash. 
+
+3. ESP8285 or ESP8266 + 1MB flash
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ESP8285 or ESP8266 + 1MB flash can use "Copy OTA Mode" for OTA, more details are in the `examples/system/ota <https://github.com/espressif/ESP8266_RTOS_SDK/tree/master/examples/system/ota/>`_.

Diff for: docs/en/general-notes/index.rst

@@ -3,11 +3,11 @@ ESP8266\_RTOS\_SDK (IDF Style) Programming Guide
 
 This is the documentation for the new `ESP8266_RTOS_SDK <https://github.com/espressif/ESP8266_RTOS_SDK>`_ which refacted to be ESP-IDF Style. ESP8266\_RTOS\_SDK is the official development framework for the `ESP8266EX <https://www.espressif.com/en/products/hardware/esp8266ex/overview>`_ chip.
 
-==================  ==================  ==================
-|Get Started|_      |API Reference|_    |API Guides|_
-------------------  ------------------  ------------------
-`Get Started`_      `API Reference`_    `API Guides`_
-==================  ==================  ==================
+==================  ==================  ==================  ==================
+|Get Started|_      |API Reference|_    |API Guides|_       |General Notes|_
+------------------  ------------------  ------------------  ------------------
+`Get Started`_      `API Reference`_    `API Guides`_       `General Notes`_
+==================  ==================  ==================  ==================
 
 .. |Get Started| image:: ../_static/get-started.gif
 .. _Get Started: get-started/index.html
@@ -18,11 +18,15 @@ This is the documentation for the new `ESP8266_RTOS_SDK <https://github.com/espr
 .. |API Guides| image:: ../_static/api-guides.gif
 .. _API Guides: api-guides/index.html
 
+.. |General Notes| image:: ../_static/general-notes.gif
+.. _General Notes: general-notes/index.html
+
 .. toctree::
    :hidden:
 
    Get Started <get-started/index>
    API Reference <api-reference/index>
    API Guides <api-guides/index>
+   General Notes <general-notes/index>
 
 * :ref:`genindex`