Update 3 files

• /README.md
• /exercises/UNIX_HPC_exercise_instructions.md
• /exercises/UNIX_HPC_cheat_sheet.md

exercises/UNIX_HPC_cheat_sheet.md

+# Bash Cheat Sheet
+
+ A cheat sheet for bash commands.
+
+
+ ## Manage and navigate directories
+
+```bash
+pwd                       # Print current directory path
+ls                        # List directories
+ls -a|--all               # List directories including hidden
+ls -l                     # List directories in long form
+ls -l -h|--human-readable # List directories in long form with human readable sizes
+cd foo                    # Go to foo sub-directory
+cd                        # Go to home directory
+cd ~                      # Go to home directory
+cd -                      # Go to last directory
+mkdir foo                 # Create foo directory
+rmdir foo                 # Delete foo directory
+rmdir -r foo              # Delete foo directory including content  
+```
+
+## Manage files
+
+```bash
+cp foo.txt bar.txt                                # Copy file
+mv foo.txt bar.txt                                # Move/Rename file
+rm foo.txt                                        # Delete foo.txt
+rm *                                              # Delete all files in current directory 
+```
+
+## Read and manipulate files
+
+```bash
+cat foo.txt            # Print all contents
+less foo.txt           # Print some contents at a time (g - go to top of file, SHIFT+g, go to bottom of file, /foo to search for 'foo')
+head foo.txt           # Print top 10 lines of file
+tail foo.txt           # Print bottom 10 lines of file
+nano foo.txt           # Simple file editor
+vi foo.txt             # Advanced file editor 
+wc foo.txt             # List number of lines words and characters in the file
+```
+
+## Input and Output
+
+```bash
+echo "Hello world!"         # Print statement to standard output
+echo "foo" > bar.txt        # Direct output into file. Overwrites file if it already exists.
+echo "foo" >> bar.txt       # Direct output into file and append if it already exists. 
+command 1 | command2        # Directs output from command1 as standard input into command2    
+```
+
+