added a README/table of contents

README.md

+# Quick References
+
+This repository contains a number of quick reference guides to various
+basic and not-so-basic topics.  These are not intended to be
+comprehensive in any way, and you will likely find your favorite
+search engine a great way to supplement this information, once you
+know the basic terms to include.
+
+Much of the information here is Linux-specific or C-specific. This is a
+feature.
+
+Here's a brief summary of what's in each file:
+
+## [admin.md](admin.md)
+
+ * The `root` administrator account
+ * Running programs as a different user
+ * Managing users and groups
+
+## [bash.md](bash.md)
+
+ * Shell variables
+ * Parameter expansion
+ * Single vs. double quotes
+ * Different ways to execute commands
+ * Positional parameters
+ * Math in bash
+ * File descriptors, like STDIN and STDOUT
+ * "Here" documents
+ * Pipelines
+ * Control flow, including conditionals and loops
+ * Functions
+ * Command aliases
+ * **Writing scripts, including the "shebang" line**
+
+## [filesystem.md](filesystem.md)
+
+ * The many ways of listing directory contents
+ * Changing directories. Number 4 may surprise you!
+ * Moving, copying, and extracting pieces of files. This includes fancy
+   commands like `dd` and `xxd`.
+ * **File permissions**, including making your scripts executable!
+ * Examining disk usage
+ * Special files, and where they live
+ * **Finding things** with `locate`, `find`, `grep`, and `ack`
+
+## [misc.md](misc.md)
+
+ * Manual pages
+ * Killing processes
+ * The `true` and `false` commands
+ * The `yes` command
+ * Generating numeric sequences (great for loops in scripts!)
+ * The `tar` command that you in theory already know
+ * Splitting lines of text with `cut`
+ * When `cut` doesn't cut it, the `awk` command
+
+## [network.md](network.md)
+
+ * Examining your network configuration (lots of commands and details)
+ * Testing connectivity with `ping` and `traceroute`
+ * Finding IP addresses for hostnames
+
+## [numbers.md](numbers.md)
+
+ * Explicitly-sized integer types, like `uint64_t`
+ * Big-endian vs. little-endian byte order
+ * Host vs. network byte order
+ * Converting between different byte orders, including a handy table!
+
+## [python.md](python.md)
+
+A brief tour of the language, which should help you orient yourself.
+This includes a complete script to generate random numbers, in
+[example.py](example.py)
+
+## [python-scapy.md](python-scapy.md)
+
+ * Capturing network packets
+ * Examining network packets
+ * Creating network packets
+
+This really only scratches the surface.