README,docs: add some documentation for the shell

doc/shell.md

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+# Using the Shell
+
+The shell is started on booting the Tridora-System, and also each
+time a program exits.
+
+Its main purpose is to start the editor and compiler, and finally run your
+compiled program.
+
+You can also list directory contents and do some file management like
+copying and deleting files.
+
+## The Menu
+The shell displays a menu with a bit of status information and a list
+of possible commands, each represented by a single letter.
+Pressing the corresponding key will invoke a specific command.
+
+Some commands will prompt for more information if needed.
+
+The menu looks like this:
+
+```
+W)orkfile: lines.pas
+V)olume:   Examples
+L)ist directory  K)runch volume  O)ther program
+D)elete file  RenaM)e file  coP)y file
+E)dit  C)ompile  A)ssemble  B)uild  R)un
+> 
+```
+
+## The Workfile
+When working with the shell, for many functions you
+need to set a _workfile_ which then will be passed as a parameter
+to other programs. In most cases, you will set the workfile to your
+source file (e.g. **hello.pas**). When appropriate, the shell will
+change the file name extension given to the called program, while the
+workfile name setting in the shell will remain the same.
+
+For example, when calling the assembler, the workfile name **hello.pas**
+will automatically changed to **hello.s**. When running the program,
+the name will be changed to **hello.prog**.
+
+## Volumes and the Default Volume
+Volumes in the Tridora-System are partitions on the SD-Card which are
+accessed by a nice name of up to 32 characters. A filename can contain
+a volume specification, which is prepended to the filename with a `#`
+character in front and a `:` at the end of the volume name. For example:
+`#Examples:lines.pas` is a file named **lines.pas** on a volume with
+the name of **Examples**.
+
+If a filename does not contain a volume specification (i.e. it does
+not start with a `#` character), the file subsystem will look for the
+file on the default volume.
+
+In the second line of the menu, the shell displays the current default
+volume. The default volume can be changed with the _V_ command.
+
+## Commands
+Below is a list and short description of each command. Pressing any
+other key will redisplay the menu.
+
+### W - Change Workfile
+Changes the workfile. To clear the workfile name, just press RETURN at
+the prompt. If you accidently pressed _W_, you can press _Control-C_
+to keep the old name.
+
+### V - Change Default Volume
+Changes the default volume. Lists all available volumes and then prompts
+for the name of the new default volume.
+Enter the name without a leading `#`. Press
+return if you do not want to change the default volume.
+
+### L - List Directory
+Lists all files on the default volume.
+
+### K - Reclaim Space and Defragment Volume
+Calls the reclaim program.
+
+The Tridora-filesystem does not automatically free up space from
+deleted files, and it needs to be defragmented occasionally.
+This is done with the _K_ command which executes the program **#SYSTEM:reclaim.prog**.
+
+First, the program scans the default volume for space used up by deleted files.
+If it finds any, it asks you if you want to reclaim that space. Press *y* to proceed
+or *n* to exit the reclaim program, returning to the shell.
+
+After reclaiming space, the program checks if there are multiple free space
+regions on the volume. If this is the case, you are asked if you want to perform
+the crunch operation, which will move files around so there are no more gaps in the
+space allocated by files. On large volumes, this can take some time, so you can
+choose if you want to proceed or not with *y* or *n*.
+
+### O - Execute Program
+This command allows you to call any program, independent of the current workfile.
+It also allows you to specify parameters.
+
+When using the _O_ command, you will be prompted for a program name. You can
+also use a file name with a volume specification if the program file is not on the
+default volume. If you omit the file extension **.prog**, it is added automatically.
+
+After entering the program name, you can specify any program arguments. You have
+to enter the arguments line by line, pressing RETURN after each argument.
+After the last argument, just press RETURN (or if you do not want to give any arguments).
+
+### D - Delete File
+Will prompt you for a file name, then will try to delete that file.
+
+### M - Rename File
+Will prompt you for an old and a new filename, then will try to perform a rename.
+You cannot use a volume specification in both filenames.
+A file with the new name must not exist.
+
+### P - Copy File
+Will prompt you for an old an a new filename, then will try to perform a copy.
+A file with the new name must not exist.
+Copying works across different volumes.
+
+The implementation of the _P_ command is inefficient, which will be noticable for larger files (>100KB).
+
+### E - Edit
+Starts the editor, editing the workfile.
+
+### C - Compile
+Calls the compiler with the workfile.
+
+### A - Assemble
+Calls the assembler with the workfile and the extension **.s**.
+
+### B - Build
+Calls the compiler with the workfile and then the assembler.
+
+### R - Run
+Runs the program compiled/assembled from the workfile, with the extension **.prog**.