improve find fqbn FAQ (arduino#274)

As reported in multiple issues (arduino#138 arduino#191 arduino#271) finding the right FQBN for a board can be difficult.

Improving the documentation may help users to find the relevant information.

Specifically, this PR closes arduino#271 and add little improvement as suggested in arduino#191.

README.md

@@ -343,8 +343,17 @@ Because:
 
 #### How can I find the core/FQBN for a board?
 
+Update the core index to have latest boards informations:
+
+    $ arduino-cli core update-index
+    Updating index: package_index.json downloaded
+
 See: https://github.com/arduino/arduino-cli#step-4-find-and-install-the-right-core
 
+Further help can be found in [this comment](https://github.com/arduino/arduino-cli/issues/138#issuecomment-459169051) in [#138](https://github.com/arduino/arduino-cli/issues/138).
+
+For a deeper understanding of how FQBN works, you should understand Arduino Hardware specification. You can find more information in this [arduino/Arduino wiki page](https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification)
+
 # Testing
 
 Currently Unit and Integration test are available for launch in 2 ways:
@@ -366,4 +375,4 @@ Currently Unit and Integration test are available for launch in 2 ways:
 * test-unit-race:       Run unit tests only with race condition detection
 ```
 
-For Example to launch unit tests only run: `task test-unit`
+For Example to launch unit tests only run: `task test-unit`