Add initial gdb content

gdb/.gitignore

+main
+main.dSYM

gdb/Makefile

+# Change your compiler here if you're not using gcc.
+CXX = g++
+
+# The flag -g adds debugging symbols to the executable. These are necessary for debugging with gdb.
+CFLAGS = -Wall -g
+
+main: main.cpp
+	$(CXX) $(CFLAGS) -o $@ $<
+
+.PHONY: clean
+clean:
+	rm main

gdb/README.md

+# GDB
+
+[GDB](https://www.gnu.org/software/gdb/) is a program that looks inside other programs. It's a
+really handy tool for tracking down hard to find bugs.
+
+## Setup
+
+First, GDB is only useful if you have a way to compile programs (usually C or C++, but GDB does
+support a variety of other languages as well). So to use it on your computer, you'll probably want
+to get set up with [gcc and make](https://gitlab.cba.mit.edu/pub/hello-world/c_cpp_and_make). For
+debugging microcontrollers, you'll need a compiler that supports your particular chip (`gcc-avr`,
+`arm-none-eabi`, etc.).
+
+To debug your microcontroller, you'll need some on-chip debugging software.
+[OpenOCD](https://gitlab.cba.mit.edu/pub/hello-world/tools/-/tree/master/openocd) is the option I'm
+familiar with.
+
+To install GDB itself, on Linux or Mac you can probably use your system's package manager (`apt` or
+`yum` on linux, [`homebrew`](https://brew.sh/) on Mac). If you want complete hacker cred, you could
+build it from [source](https://www.gnu.org/software/gdb/current/).
+
+## Debugging on Your Computer
+
+## Debugging on Your Microcontroller
+
+I'll refrain from

gdb/main.cpp

+#include <iostream>
+
+// This is a simple function that checks if numbers are prime. Or does it? There may be a bug...
+bool is_prime(int x) {
+    for (int i = 2; i < x / 2; ++i) {
+        if (x % i == 0) {
+            return false;
+        }
+    }
+    return true;
+}
+
+int main(int argc, const char* argv[]) {
+    // If the user didn't give us a number, exit.
+    if (argc < 2) {
+        printf("Please type a number.\n");
+        return 0;
+    }
+    int x = atoi(argv[1]);
+
+    bool const prime = is_prime(x);
+    if (prime) {
+        std::cout << x << " is prime\n";
+    } else {
+        std::cout << x << " is not prime\n";
+    }
+
+    return 0;
+}