CLI tutorials (#642)

* Initial CLI tutorials

Author: cholmes

docs/cli-concepts.md

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+# Command-line Interface (CLI) Concepts
+
+Planet's command-line interface is built of composable pieces that combine in powerful ways.
+To take full advantage of all that Planet's CLI offers there are a few concepts and tools
+that are worth explaining. 
+
+**NOTE:** This document is still a work in progress, but we'll work to flesh it out. We'll
+leave it up since there's still a bit of good information.
+
+## Core Unix Concepts
+
+If you are completely new to the command-line we recommend starting with a real introductory
+guide like (TODO, link to a good guide). But we wanted to go over a few key concepts that 
+are a bit more 'advanced', as they allow you to get the most out of Planet's CLI. These are
+all built into any unix command-line, including Linux and the Mac terminal. If you're on 
+Windows you can use [Windows Subsytem for Linux](https://docs.microsoft.com/en-us/windows/wsl/about)
+or [Cygwin](https://www.cygwin.com/).
+
+### Piping & Redirection
+
+Several commands in the Planet CLI are used to create files that are used as input to 
+other commands. The default of these commands is to just print the output on your screen.
+Seeing it on the screen can be useful for making sure it's right, but you'll most likely
+want to make use of it. This is where the concept of 'redirection' comes in. If you use the 
+`>` character and then specify a file name the command-line will save its output to that file.
+So if you say:
+
+```
+ planet data filter --range cloud_percent lt 10 > filter.json
+```
+
+Then the output will be saved. This output is referred to as STDOUT, or 'standard output'. 
+There is much more in this vein that you can do, like use `>>` to append to an existing
+file, or `<` to send what's in the file as input for a command. 
+
+One of the most powerful concepts that we use extensively in the Planet CLI is 'piping'. 
+The `|` is the pipe symbol, and it's a special command that lets you pass the output from
+one command to be the input for the next one. So instead of having to save to a file and
+then referring to it you can just do it all in one call:
+
+```
+planet data filter --range cloud_percent lt 10 | planet data search-quick PSScene -
+```
+
+The pipe says to take the output of the first command and pass it to the input of 
+the second. You'll notice that the planet command has a dash (`-`), this is a convention
+that is often used by different CLI programs to explicitly say 'read from
+standard out'. Most Planet CLI commands require it, but one or two will implicitly
+read from standard out if it's not explicitly included. Using the dash to mean
+'read from standard out' is a general convention used by many programs, but it's 
+not universal, so check the docs of the program you're using as to how it reads 
+from piped input. For example GDAL/OGR uses a specific `/vsistdin/` convention to 
+read from a pipe.
+
+If you'd like to learn more about these topics then check out 
+[this tutorial](https://ryanstutorials.net/linuxtutorial/piping.php). And if you'd
+like to learn more about the dash (`-`) see 
+[this tutorial](https://www.baeldung.com/linux/dash-in-command-line-parameters).
+
+
+### head & tail
+
+Also less/more?
+
+### Running a command within another
+
+### variables
+
+## JQ
+
+## curl
+
+## sed
+