Instructions in README.md require clarification

[aliphys]

The current README.md provide a high level overview on how to get started with using the Arduino_Cellular library.

While they may be clear to someone already familiar with the platform, they require clarification to be accessible to a wider audience and ensure people can get started quickly and easily. Some specific points:

• The SIM card size is not mentioned. From the picture, it is not clear if it a normal, micro or nano SIM card should be used.
  • 
• Library operation is dependent upon the configuration of PCIE
• The term either here is unclear, which may refer to directly connecting the SIM card to the PRO 4G Module. Underneath the PRO 4G Modules, there is a silkscreen of a SIM card with the contacts exposed. However, this should not be the first point of entry for users

  Arduino_Cellular/README.md

  Line 28 in 414d5e3

  2. Insert a valid SIM card either in the **PCIE_SIM** connector on the Portenta Mid Carrier

• It is requested that header pins are connected, for both the power header and the PCIE breakout. The connector names are not mentioned, which makes it difficult to cross-reference with the datasheet for the Portenta Mid Carrier.
• Connecting the microUFL antennas is not stated, not is it mentioned where to obtain them. They are not included by default in the EMEA nor GNSS versions of the PRO 4G Modules. A reference to the Arduino Pro 4G Module Antennas Kit should be given.
• It is not made clear where or how the external power supply should be connected. Given that it is stated that VIN is 5V, and that USB is commonly 5V, it may be safe to assume that this refers to the USB-C power supply. However, according to developers the external power source is distinct from the USB power supply.

  Arduino_Cellular/README.md

  Line 33 in 414d5e3

  5. Connect external power to the Mid Carrier, via the **VIN** (5V) because modems use a lot of power when connecting or getting a GPS location. Make sure your supply can handle around 3A.

• In the Instructions, variables are created for storing secrets while the examples use the #define compiler directive. This will cause confusion.

  • Arduino_Cellular/README.md

    Lines 35 to 39 in 414d5e3

    	```cpp
    	const char apn[] = "live.vodafone.com";
    	const char gprsUser[] = "live";
    	const char gprsPass[] = "";
    	```

  • https://github.com/aliphys/Arduino_Cellular/blob/414d5e33bcc4f10eb87fe8a3f60eff54f773275a/examples/HTTPClient/arduino_secrets.h#L1-L4