Add doc, remove obsolete file

xivsupport/src/main/java/gg/xp/xivsupport/gui/map/omen/README.md

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+# About Cast Locations
+
+Cast locations are not particularly straightforward.
+
+## Instant Casts
+
+Let's take an instant cast skill. We get a 21/22-line, and a 264-line.
+
+If the skill is ground-targeted, we will get a 264-line with a specific cast location and rotation.
+This is the best possible data and is the best case scenario. In this case, we need no other information
+from the log lines.
+
+However, most skills will not have this. We use two other pieces of data:
+- The cast angle
+- The animation target
+
+The animation target is useful for determining, for example, whether an instant cast AoE is centered on
+the caster, or the target (if one exists). For example, Sage's Phlegma is centered on the target, but
+Dyskrasia is centered around the caster.
+
+If we do not have those pieces of information, we try to guess based on the CastType.
+
+## Non-Instant Casts
+
+We also want to display an omen on skills that are still casting.
+
+Like the instant cast skills, if we have a location and rotation,
+that is the best case scenario. We need no other data.
+
+If the skill is not instant, we use the target of the cast and rotation
+to try to determine it. We assume that if the skill is AoE, and has a target,
+that it will use that target as the basis for the aoe.
+
+If the skill is not instant, and we can associate it back to a cast (20-line),
+and the cast event has a target other than environment or the caster,
+then we can assume that the targeted entity is the true aoe target.
+
+# Cast AoEs
+
+The actual cast shape and size is determined mainly by three properties of the action:
+- CastType: determines shape of cast (see `OmenType.fromCastType(int)`)
+- EffectRange: determines the primary size (e.g. circle radius or rectangle length)
+- XAxisModifier: determines the secondary size (rectangle width, unused for circles and cones)
+
+Unfortunately, there are two data points that cannot be derived from this data alone:
+- Inner radius of donuts
+- Angle of cones
+
+For the inner donut radius, often times it is simply the caster's hitbox, but not always.
+For the cone angle, if the skill has an omen (technical term for skill telegraph), it can 
+sometimes be derived from the name of the "omen" . For example, Action 21471 (Wingbeat) 
+has an omen named "er_gl_fan060_1bf". From the "060" part, we can assume that the angle is
+60 degrees.