Add interrupt section to docs (#6560)

* Add info about installing python3 on Mac, Linux

The Mac requires special handholding to allow SSL connections for
get.py, so document those for end users.

* Add interrupt section to docs

Adds a section on interrupts to the docs and lists the restrictions
and warnings about running long tasks.

Fixes #6428

* Update per review comments

doc/reference.rst

@@ -1,6 +1,42 @@
 Reference
 =========
 
+Interrupts
+----------
+
+Interrupts can be used on the ESP8266, but they must be used with care
+and have several limitations:
+
+* Interrupt callback functions must be in IRAM, because the flash may be
+  in the middle of other operations when they occur.  Do this by adding
+  the ``ICACHE_RAM_ATTR`` attribute on the function definition.  If this
+  attribute is not present, the sketch will crash when it attempts to
+  ``attachInterrupt`` with an error message.  
+
+.. code:: cpp
+
+    ICACHE_RAM_ATTR void gpio_change_handler(void *data) {...
+
+* Interrupts must not call ``delay()`` or ``yield()``, or call any routines
+  which internally use ``delay()`` or ``yield()`` either.
+  
+* Long-running (>1ms) tasks in interrupts will cause instabilty or crashes.
+  WiFi and other portions of the core can become unstable if interrupts
+  are blocked by a long-running interrupt.  If you have much to do, you can
+  set a volatile global flag that your main ``loop()`` can check each pass
+  or use a scheduled function (which will be called outside of the interrupt
+  context when it is safe) to do long-running work.
+
+* Memory operations can be dangerous and should be avoided in interrupts.
+  Calls to ``new`` or ``malloc`` should be minimized because they may require
+  a long running time if memory is fragmented.  Calls to ``realloc`` and
+  ``free`` must NEVER be called.  Using any routines or objects which call
+  ``free`` or ``realloc`` themselves is also forbidden for the same reason.
+  This means that ``String``, ``std::string``, ``std::vector`` and other
+  classes which use contiguous memory that may be resized must be used with
+  extreme care (ensuring strings aren't changed, vector elements aren't
+  added, etc.).
+
 Digital IO
 ----------