Clarify dependency of Array Merge on Mergesort

This adds some text in the README that attempts to clarify the dependency of the Array Merge part of the lab on Mergesort.

Author: NicMcPhee

README.md

@@ -129,6 +129,12 @@ Some things to watch our for:
 
 There are more comprehensive tips and suggestions in `Tips_and_suggestions.md` in the repository.
 
+:bangbang: The Array Merge problem depends on the Mergesort problem, so you really
+should do Mergesort first. The GitHub Actions (that run the tests automatically and
+create things like the status badges) actually include `mergesort.c` when they
+compile the Array Merge code, so if `mergesort.c` is incomplete or otherwise
+broken that will cause the Array Merge tests to fail.
+
 ### Mergesort
 
 Your task here is to implement a well known sorting algorithm in C,
@@ -222,7 +228,22 @@ the array without messing with that important first element.
 You can use references like `../mergesort/mergesort.h` or
 `../mergesort/mergesort.c` to access appropriate files in that part of
 the project, either in things like `#include` statements or
-in `gcc/g++` calls.
+in `gcc/g++` calls. The command in the GitHub Actions to compile
+the Array Merge tess, for example, is:
+
+```text
+g++ -Wall -g -o array_merge_test ../mergesort/mergesort.c array_merge.c array_merge_test.cpp -lgtest -pthread -std=c++0x
+```
+
+Note that the `../mergesort/mergesort.c` in this command includes
+the Mergesort `.c` file in the list of files that will be compiled
+and linked together to form the `array_merge_test` executable.
+That has two major implications:
+
+- You *must* have a working `mergesort.c` in order to get the GitHub
+  Actions for Array Merge to compile successfully.
+- You *can* (and presumably should?) use the function `mergesort()` that
+  you define in the other part of the lab here in Array Merge.
 
 ## Final Words