Add resizing to contributing and drive-by format

This PR adds information on how to resize your images when
contributing.

It also applies a few formatting changes such as:

- Using named links to reduce clutter
- Folding the text at 80 characters for command line users
- Removing dollar signs in codeblocks if no command output is provided
  [MD014][md014]

[md014]: https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md014---dollar-signs-used-before-commands-without-showing-output

CONTRIBUTING.md

@@ -1,33 +1,57 @@
 # Contributiton Guide
 
-You shouldn't edit the node class files (all files under `diagram` directory) by yourself.
+You shouldn't edit the node class files (all files under `diagram` directory) by
+yourself.
 
 ## Resources
 
 ### Update nodes
 
-All node classes was auto-generated from image resource files. For example, the `diagram.aws.compute.EC2` class was auto-generated based on `resources/aws/compute/ec2.png` image resource file.
+All node classes was auto-generated from image resource files. For example, the
+`diagram.aws.compute.EC2` class was auto-generated based on
+`resources/aws/compute/ec2.png` image resource file.
 
-So, if you want to add new node resources or update existing node resources, you can just add or update the image files in `resources/<provider>/<type>/<image>`.
+So, if you want to add new node resources or update existing node resources, you
+can just add or update the image files in `resources/<provider>/<type>/<image>`.
+
+Images should be resized to fit a maximum of 256 pixels wide or high.
+
+```shell
+# You can do that easily with ImageMagick
+convert -resize 256 my_big_image.jpg my_image.jpg
+
+# Or FFmpeg
+ffmpeg -i my_big_image.jpg -vf scale=w=256:h=256:force_original_aspect_ratio=decrease my_image.png
+```
 
 Then just run the `./autogen.sh` to generate the added or updated node classes.
 
-> IMPORTANT NOTE: To run `autogen.sh`, you need [round](https://github.com/mingrammer/round) and [inkscape](https://inkscape.org/ko/release) command lines that are used for clearning the image resource filenames.
+> IMPORTANT NOTE: To run `autogen.sh`, you need [round][round] and
+> [inkscape][inkscape] command lines that are used for clearning the image
+> resource filenames.
 >
 > macOS users can download the inkscape via Homebrew.
 
+[round]: https://github.com/mingrammer/round
+[inkscape]: https://inkscape.org/ko/release
+
 ### Update Aliases
 
-Some node classes have alias. For example, `aws.compute.ECS` class is an alias of `aws.compute.ElasticContainerService` class. Aliases also were auto-generated from `ALIASES` map in [config.py](config.py).
+Some node classes have alias. For example, `aws.compute.ECS` class is an alias
+of `aws.compute.ElasticContainerService` class. Aliases also were auto-generated
+from `ALIASES` map in [config.py](config.py).
 
-So, if you want to add new aliases or update existing aliases, you can just add or update the `ALIASES` map in [config.py](config.py). 
+So, if you want to add new aliases or update existing aliases, you can just add
+or update the `ALIASES` map in [config.py](config.py).
 
 Then just run the `./autogen.sh` to generate the added or updated aliases.
 
-> IMPORTANT NOTE: To run `autogen.sh`, you need [round](https://github.com/mingrammer/round) and [inkscape](https://inkscape.org/ko/release) command lines that are used for clearning the image resource filenames.
+> IMPORTANT NOTE: To run `autogen.sh`, you need [round][round] and
+> [inkscape][inkscape] command lines that are used for clearning the image
+> resource filenames.
 
 ## Run Tests
 
 ```shell
-$ python -m unittest tests/*.py -v
+python -m unittest tests/*.py -v
 ```