Readme update: lfclk implications, SoftDevice selection (sandeepmistry#151)

* clarify effects of lfclk selection

As per Softdevice changelog "Synthesized low frequency clock source is not tested or intended for use with BLE stack."
Also https://devzone.nordicsemi.com/question/953/what-low-frequency-clock-sources-can-i-use/?answer=955#post-id-955
Maybe option should better be removed?

* implications of soft device selection

* add S132, adhere SoC to naming, add versions

according to SoftDevice releasenotes

* incorporate suggestions

Author: Suschman

README.md

@@ -74,6 +74,12 @@ For 64-bit Linux users,  ```libc6:i386```, ```libstdc++6:i386```, ```libncurses5
 
 __NOTE__: To roll back to the original driver go to: Device Manager -> Right click on device -> Check box for "Delete the driver software for this device" and click Uninstall
 
+### Selecting a SoftDevice
+SoftDevices contain the BLE stack and housekeeping, and must be downloaded once before a sketch using BLE can be loaded.
+The SD consumes ~5k of Ram + some extra based on actual BLE configuration.
+* SoftDevice S110 v8.0.0 supports [Revision](http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf51%2Fdita%2Fnrf51%2Fcompatibility_matrix%2FnRF51822_ic_revision_overview.html&cp=3_0_1) 2 and 3 of nRF51 in peripheral role. It is 96k in size.
+* SoftDevice S130 v2.0.1 supports Revision 3 of nRF51 in peripheral and central role. It is 108k in size.
+* SoftDevice S132 v2.0.1 supports nRF52 in peripheral and central role. It is 112k in size.
 
 ### Flashing a SoftDevice
 
@@ -117,7 +123,7 @@ This Arduino Core does **not** contain any Arduino style API's for BLE functiona
 
 If the selected board has an external 32 kHz crystal connected, it will be used as the source for the low frequency clock. Otherwise the internal 32 kHz RC oscillator will be used. The low frequency clock is used by the `delay(ms)` and `millis()` Arduino API's.
 
-The Generic nRF51 and nRF52 board options have an additional menu item under `Tools -> Low Frequency Clock` that allows you to select the low frequency clock source.
+The Generic nRF51 and nRF52 board options have an additional menu item under `Tools -> Low Frequency Clock` that allows you to select the low frequency clock source. However, Nordic does not recommend the Synthesized clock, which also has a significant power impact.
 
 ## Credits