Docs: add an entry about compiler warnings

doc/faq/a02-my-esp-crashes.rst

@@ -6,6 +6,7 @@ My ESP crashes running some code. How to troubleshoot it?
 -  `Introduction <#introduction>`__
 -  `What ESP has to Say <#what-esp-has-to-say>`__
 -  `Get Your H/W Right <#get-your-hw-right>`__
+-  `Enable compilation warnings <#enable-compilation-warnings>`__
 -  `What is the Cause of Restart? <#what-is-the-cause-of-restart>`__
 -  `Exception <#exception>`__
 -  `Watchdog <#watchdog>`__
@@ -73,6 +74,43 @@ In any case, make sure that your module is able to stably run standard
 example sketches that establish Wi-Fi connection like, e.g.,
 `HelloServer.ino <https://github.com/esp8266/Arduino/tree/master/libraries/ESP8266WebServer/examples/HelloServer>`__.
 
+Enable compilation warnings
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Most common issues may be resolved by enabling compilation warnings and fixing them.
+
+For Arduino IDE, select ``File -> Preferences``:
+
+- Make sure ``Show verbose output during: compilation`` is enabled
+- Set ``Compiler warnings`` to ``More`` or ``All``
+
+For PlatformIO, all warnings should already be enabled by default.
+
+Notice that the default configuration of Arduino IDE inhibits **all** compilation warnings.
+For the ESP8266 platform, some warnings should be treated as errors, otherwise it may cause unexpected issues at runtime:
+
+.. code:: cpp
+
+    int func() {
+    }
+
+    int other() {
+      return func();
+    }
+
+Should fail to build with the following message:
+
+.. code:: console
+
+    return-value.cpp: In function ‘int func()’:
+    return-value.cpp:2:1: error: no return statement in function returning non-void [-Werror=return-type]
+        2 | }
+          | ^
+    compilation terminated due to -Wfatal-errors.
+    cc1plus: some warnings being treated as errors
+
+Notice that ``-Werror=return-type`` is the default starting with Core 3.0.2 w/ GCC 10.3
+
 What is the Cause of Restart?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~