btrfs-progs: docs: document command line conventions

First draft of the command line conventions regarding options, wording,
style and formatting.

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/CmdLineConventions

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+# Command line, formatting, UI guidelines
+
+## Command line options
+
+### Short options
+
+* reserved for the most common operations
+* should follow some common scheme
+ * verbose, recursive, force, output redirection, ...
+ * same option means the same thing in a group of related options
+* most have an equivalent long option
+
+### Long options
+
+* short but descriptive
+* long-worded long options are acceptable for rare but seemingly unique operations
+  * example: `btrfs check --clear-space-cache v1`
+
+### Help text
+
+* short help for *--help* output: `btrfs subcommand [options] param1 param2`
+ * the number of options gets unwieldy for many options so it's better to
+   insert the stub and document properly all of them in the detailed section
+* short description after command line: terse but understandable explanation
+  what the command does
+* long description after the short description
+ * explain in more detail what the command does, different use cases, things to notice
+ * more complex things should be documented in the manual pages and pointed to
+
+## Command output, verbosity
+
+### Structured output
+
+* `Key: value`
+* indentation used for visual separation
+* value column alignment for quick skimming
+* must be parseable by scripts but primary consumer of the output is a human, and greppable for logs
+
+### Default output
+
+* unix commands do one thing and say nothing, we may diverge a bit from that
+* default output is one line shortly describing the action
+ * why: running commands from scripts or among many other commands should be
+   noticeable due to progres tracking or for analysis if something goes wrong
+ * there's a global option to make the commands silent `btrfs -q subcommand`,
+   this can be easily scripted eg. storing the global verbosity option in a
+   variable, `btrfs $quiet subcommand` and then enabling or disabling as needed
+* line length should not exceed 80 columns if possible but this is not a strict
+  limit and we want to put the relevant information there rather abbreviate too
+  much
+
+### Formatting
+
+* numeric values are not quoted
+* string values are quoted if they would be confused when parsed word by word
+  (eg. file paths)
+  * quoting is by single apostrophe on both ends
+* all messages follow a few known and understood schemes
+
+### Verbosity levels
+
+* 0 - quiet, only errors to stderr, nothing to stdout
+* 1 - default, one line, see above
+* 2 - inform about main steps that happen
+* 3 - a little bit more details about what happens during level 2
+* 4 - lots of information, for debugging and close inspection what happens