misc: Added a HACKING.md to provide an explanation of nomenclature and how we handle reset terminology

src/target/HACKING.md

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+# Information and terminology guide
+
+## Reset nomenclature
+
+Within this code base, we refer to the physical reset pin of a target device by 'nRST'/'nRESET'.
+This is because while originally primrily an ARM debugger, the project is now also a debugger for
+RISC-V and AVR where the ARM ADIv5 terminology 'SRST' meaning 'System reset' is ambiguous and
+oft very confusing.
+
+The history of this is that 'SRST' in AVR and RISC-V mean 'Software Reset' - by which we refer
+to a reset initiated over JTAG protocol by poking registers in the target device.
+ADIv5 'System Reset' refers to resetting the target using its nRST pin.
+
+A third moving piece to all this is that 'nTRST' (sometimes also referred to as 'JRST') is an
+optional pin sometimes found in JTAG interfaces to reset the JTAG machinary itself.
+
+In summary, the following applies:
+
+* In the ADIv5 spec, the physical reset pin is referred to by 'SRST'
+* In this code base, we refer to it by 'nRST'
+* Refering to it by 'nRST' is also then consistent with the silkscreen naming convention
+  on probe host boards and in the ARM 10-pin JTAG pinout
+* JTAG physical reset is referred to by 'nTRST'
+* Software reset as in the case of JTAG-PDI is refered to by 'SRST' if shortened.
+
+The upshot of this is that to inhibit physical reset in the ARM ADIv5/Cortex-M code, set
+`CORTEXM_TOPT_INHIBIT_NRST`, which refers to inhibiting the ADIv5 spec 'SRST'.