Extensive modifications to overview.md (#957)

* Extensive modifications to overview.md.

I tried to add clarity and details while maintaining the spirit of the original
document. However, it might be a bit too 'dry' now. I'd be happy to liven it up
a bit. I'm only serious when I feel like I _have_ to be.

One of my opinions (which may be at odds with yours) is that the documentation
should emphasize `jrnl`'s advantages without downplaying any other existing
solutions.

If I have time, I'd like to add more information about the documented benefits
of journaling, particularly the mental health aspects. That will probably need
its own page, but I'm new here, so I don't want to overstay my welcome. :)

* More changes to overview.md in response to feedback

"why keep a journal" section removed -- it could be re-added as its own page,
but it's not front-page material

* More changes to overview.md:
- fixed up the headings
- added information about multi-platform support

This is a work in progress and _not_ ready for prime-time.
- need to add to Command-Line Interface section, among other things

* More changes to overview.md:

- moved "`jrnl` is a simple..." to "Command-Line Interface" section

docs/overview.md

@@ -1,27 +1,41 @@
 # Overview
 
-## What is jrnl?
+## Features
 
-`jrnl` is a simple journal application for
+### Command-Line Interface
-your command line. Journals are stored as human readable plain text
-files - you can put them into a Dropbox folder for instant syncing and
-you can be assured that your journal will still be readable in 2050,
-when all your fancy iPad journal applications will long be forgotten.
 
-Optionally, your journal can be encrypted using the [256-bit
+`jrnl` is a simple but powerful plain text journal application for the command
-AES](http://en.wikipedia.org/wiki/Advanced_Encryption_Standard).
+line. Everything happens on the command line.
 
-## Why keep a journal?
+### Text-Based
 
-Journals aren't just for people who have too much
+`jrnl` stores your journals as human-readable, future-proof plain text files.
-time on their summer vacation. A journal helps you to keep track of the
+You can store them wherever you want, including in shared folders to keep them
-things you get done and how you did them. Your imagination may be
+synchronized between devices. And because journal files are stored as plain
-limitless, but your memory isn't.
+text, you can rest assured that your journals will be readable for centuries.
 
-For personal use, make it a good habit to write at least 20 words a day.
+### Support for Multiple Journals
-Just to reflect what made this day special, why you haven't wasted it.
+  
+`jrnl` allows you to work with multiple journals, each of which is stored as a
+single file using date and time tags to identify individual entries. `jrnl`
+makes it easy to find the entries you want, and only the ones you want, so that
+you can read them or edit them.
 
-For professional use, consider a text-based journal to be the perfect
+### Support for External Editors
-complement to your GTD todo list - a documentation of what and how
+
-you've done it. Or use it as a quick way to keep a change log. Or use it
+`jrnl` allows you to search for specific entries and edit them in your favorite
-to keep a lab book.
+text editor.
+
+### Encryption
+  
+`jrnl` includes support for [256-bit AES
+encryption](http://en.wikipedia.org/wiki/Advanced_Encryption_Standard) using
+[cryptography.io](https://cryptography.io).
+
+### Multi-Platform Support
+
+`jrnl` is compatible with most operating systems. Pre-compiled binaries are available through several distribution channels, and you can build from source. See the installation page for more information.
+
+### Open-Source
+
+`jrnl` is written in [Python](https://www.python.org) and maintained by a [friendly community](https://github.com/jrnl-org/jrnl) of open-source software enthusiasts.