Update readme

README.md

@@ -4,6 +4,50 @@
 
 This crate is named for the Dark Lord Sauron. The name is taken from the Quenya for 'Lord of Gifts,' which has a hilarious mispronunciation that seems to apply almost directly to this case. Like the library it's based on, this is clearly for making evil things.
 
+## Usage
+
+```shell
+annatar foo.jpg \
+    --top "This text will appear near the top of the image." \
+    --middle "This text will appear near the middle of the image." \
+    --bottom "This text will appear near the bottom of the image."
+```
+
+Per the usual conventions, `-t`, `-m`, and `-b` are also available as arguments. Additionally, a `-c --caption` argument is available as a synonym for `-b --bottom`.
+
+Images may be provided as either local paths or URLs; annatar is happy to fetch your picture from the internet for you.
+
+### Annotation size
+
+By default, annatar sizes the text used for your captions on the basis of the height of the image itself. The exact algorithm used for this purpose was selected by a team of scientists working round the clock for weeks on end at the Vatican, and we didn't let them out until we saw white smoke. Rumors that the members of our text scaling enclave were able to agree only once the majority of members had starved or been bludgeoned to death by the others are, as far as you know, unfounded.
+
+The important thing is that, normally, the text will look ok. For images with strange aspect ratios (either very wide or very narrow relative to their height), text can look either too large or too small. In that case, or in the case wherein you prefer to express greater emphasis, you may prefer to pass the `-s --scale` flag with a scaling multiplier.
+
+```shell
+annatar doge.png \
+    --scale 2.0 \
+    --top "SUCH BIG" \
+    --bottom "SO SCALE"
+```
+
+This scale multiplier acts (surprisingly) as a *multiplier* for the scaling value selected by annatar. So, text scaled at `2.0` will be twice as tall (annatar scales text by height, proportionally) as it would have been otherwise.
+
+> Note: You will probably find a value like `2.0` to be excessive under most circumstances; I usually scale by about 30%—`0.7` or `1.3`—at most.
+
+#### In-band annotation scaling
+
+The `-s --scale` mutliplier is set for *all* annotations, top, middle, and bottom. To allow annotations of different size, an in-band scaling format is provided.
+
+```shell
+annatar doge.png \
+    --top "this one is normal" \
+    --bottom "\1.3 this one is bigger!"
+```
+
+White space between the scaling modifier (`\1.3` above) and the annotation (`this one is bigger`) will be ignored. However, some amount of intervening white space *is required.*
+
+> Note: for those of you who are plagued by morbid curiosity, here's the regular expression used: `\\(?P<scale>\d+(\.\d+)?)\s+(?P<caption>.+)`.
+
 ## License
 
 Licensed under either of