Add an overview/intro for the Library API

Doxyfile

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 PROJECT_NAME           = "IRremoteESP8266"
 OUTPUT_DIRECTORY       = docs/doxygen
 INPUT                  = src
+INPUT                  += docs/doxygen_index.md
 RECURSIVE              = YES
 EXCLUDE                = examples
 MULTILINE_CPP_IS_BRIEF = YES

docs/doxygen_index.md

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+# IRremoteESP8266 Library API Documentation {#mainpage}
+
+## Getting Started
+
+### The basics
+For sending messages, look at the IRsend class.
+
+For receiving messages, look at the IRrecv & decode_results classes.
+
+### Air Conditioners
+For _generic_ Air Conditioner control, look at the IRac class & the
+stdAc::state_t structure.
+
+For _detailed_ Air Conditioner control, you need to determine what protocol the
+library detects your remote/Air Conditioner to be, look into the appropriate
+`src/ir_Protocol.[h|cpp]` files and use the appropriate class object.
+e.g. if `IRrecvDumpV2` (or better) detects the protocol as `KELVINATOR`,
+open the `src/ir_Kelvinator.*` files, and examine the IRKelvinatorAC class the
+methods available to create/decode/send `KELVINATOR` messages with all the
+abilities the library offers. You can also select it from the
+[Classes](annotated.html) menu above.
+
+Various native constants & options for a given Protocol's class object can be
+found in the associated header file for that protocol.
+
+## Examples
+Most of the common uses of this library's APIs have demonstration code
+available under the [examples](https://github.com/crankyoldgit/IRremoteESP8266/tree/master/examples)
+directory. It ranges from trivial examples to complex real-world project code.
+
+## Tuning
+The most commonly used & needed knobs for controlling aspects of this library
+are available via run-time class methods or at class-object instantiation.
+Again, you are referred to the IRsend & IRrecv classes.
+
+### Advanced
+Certain addition constants and options are available as compile-time tweaks.
+You should inspect [IRremoteESP8266.h](https://github.com/crankyoldgit/IRremoteESP8266/blob/master/src/IRremoteESP8266.h),
+[IRsend.h](https://github.com/crankyoldgit/IRremoteESP8266/blob/master/src/IRsend.h),
+& [IRrecv.h](https://github.com/crankyoldgit/IRremoteESP8266/blob/master/src/IRrecv.h)
+for General, Sending, & Receiving tweaks respectively.
+
+#### Protocol timings
+Generally you should never need to adjust the timing parameters for a given
+protocol or device. However, occasionally some individual devices just want to
+be special.
+If you are having problems decoding/receiving a message, look into the
+`tolerance`, `kTolerance`, or IRrecv::setTolerance constants/methods etc first.
+However, if your problems is sending, or adjusting the tolerance doesn't work
+you may need to tweak time per-protocol timing values. These are stored as
+constants in the `ir_ProtocolName.cpp` file for the given protocol. This is
+typically a path/action of last resort.
+
+#### Reducing flash & code usage.
+You can disable most protocols by either modifying the appropriate `#define`s
+in [IRremoteESP8266.h](https://github.com/crankyoldgit/IRremoteESP8266/blob/master/src/IRremoteESP8266.h)
+or passing the appropriate compile-time flags, as documented in the same file.
+
+Avoid using the A/C classes, especially the IRac class as they will force the
+compiler to include large amounts of code you may not need.