README file for NRF52 HAL implementation

Added SPI and I2C section.

Author: Marcus Chang

targets/TARGET_NORDIC/TARGET_NRF5x/README.md

@@ -0,0 +1,42 @@
+## Mbed HAL Implementation Details
+
+### SPI and I2C
+
+The TWI/TWIM (I2C) and SPI/SPIM module shares the same underlying hardware and each instance can only provide one functionality at a time. Both the NRF52832 and NRF52840 have 2 TWI/TWIM modules and 3 SPI/SPIM:
+
+| Instance 0 | Instance 1 | Instance 2 |
+| :---:      | :---:      | :---:      |
+| SPI0/SPIM0 | SPI1/SPIM1 | SPI2/SPIM2 |
+| TWI0/TWIM0 | TWI1/TWIM1 |            |
+
+When instantiating a new Mbed SPI object or I2C object, the object will be assigned a hardware instance. By default, the HAL implementation will automatically pick a hardware instance based on the assigned pins in the following order:
+
+1. The driver will look up the pins in the configuration table and pick a pre-assigned instance.
+1. If the pins can't be found in the configuration table, the driver will check if a hardware instance has already been assigned to those pins so that objects using the same pins will share the same instance.
+1. If no instance has been assigned, the driver will look for a free instane. For I2C objects instances are assigned from 0 to 1. For SPI objects instances are assigned from 2 to 0. This ensures that no matter the order of instantiation the first three objects can be guaranteed to be on separate instances no matter the pins being used. 
+1. If no unused instance can be found, objects not sharing any pins will be assigned to the same default instance, which is `Instance 0` for I2C and `Instance 2` for SPI.
+
+#### Customization
+
+A custom configuration table can be provided by overriding the weakly defined default empty table. In the example below, I2C objects using pins `p1` and `p2` for `SDA` and `SCL` will always be assigned to `Instance 1` and SPI objects using pins `p3`, `p4`, `p5` for `MOSI`, `MISO`, and `CLOCK` will be assigned to `Instance 2` and SPI objects using pins `p6`, `p7`, and `p8` will be assigned to `Instance 0`. The custom configuration table must always be terminated with a row of `NC`.
+
+```
+const PinMapI2C PinMap_I2C[] = { 
+    {p1, p2, 1},
+    {NC, NC, NC}
+};
+
+const PinMapSPI PinMap_SPI[] = {
+    {p3, p4, p5, 2},
+    {p6, p7, p8, 0},
+    {NC, NC, NC, NC}
+};
+```
+
+The tables must be placed in a C compilation file.
+
+#### Concurrency
+
+1. When called from the same thread, it is safe to assign I2C and SPI objects to the same instance. 
+1. If an instance is being used exclusively for either I2C or SPI, the objects can safely be called from multiple threads.
+1. If an instance is being used for both I2C and SPI, the user must provide thread safety between the objects.