feat: add explanation for MCOPY

specs/opcode/94MCOPY.md

@@ -1,5 +1,15 @@
 # MCOPY opcode
 
+## Procedure
+
+The `MCOPY` opcode pops `dest_offset`, `offset` and `size` from the stack.
+It then copies `size` bytes in the current environment from memory at an `offset` to the memory at a `dest_offset`. For out-of-bounds scenarios - the memory is extended with respective gas cost applied.
+
+The gas cost of `MCOPY` opcode consists of two parts:
+
+1. A constant gas cost: `3 gas`
+2. A dynamic gas cost: cost of memory expansion and copying (variable depending on the `size` copied to memory)
+
 ## Constraints
 
 1. OpcodeId check
@@ -19,5 +29,5 @@
 
 ## Exceptions
 
-1.
-2.
+1. stack underflow: `1022 <= stack_pointer <= 1024`
+2. out of gas: remaining gas is not enough