Add Grepsedawk's Sniper String Documentation (#3)

Author: grepsedawk

docs/operations/sniper.md

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+# Grepsedawk's Sniper String (Beta)
+
+This sniper string is based off of BilisOnyxia's very popular sniper string, but rather than having hard
+price brackets, it applies a smooth curve to the values.
+
+<img alt="Graph of Grepsedawk's Price String vs BilisOnyxia's Sniper String" src=/images/sniper-graph.png width=50%/>
+
+## Custom Sources (Required!)
+
+It requires the same `minprice` custom source as BilisOnyxia's string which is as follows: 
+
+```tsm
+max(min(dbhistorical, dbregionmarketavg, dbregionhistorical), vendorsell)
+```
+
+## Sniper String
+
+```tsm
+ifgte(itemquality, 1, ifgte(minprice, 1000g, minprice * min(5 * (minprice / 10000) ^ (0.22), 80) / 100, 0c), 0c)
+```
+
+It takes all non-grey items that have a `minprice` > 1000g and applies the curve as pictured in the graph with a ceiling of 80% minprice