Add docs. Default vindex is now xxhash

Signed-off-by: Rohit Nayak <rohit@planetscale.com>

tools/map-shard-for-value/map-shard-for-value.go

@@ -129,7 +129,7 @@ func generateUniformShardRanges(totalShards int) string {
 }
 
 func main() {
-	vindexName := flag.String("vindex", "hash", "name of the vindex")
+	vindexName := flag.String("vindex", "xxhash", "name of the vindex")
 	shardsCSV := flag.String("shards", "", "comma-separated list of shard ranges")
 	totalShards := flag.Int("total_shards", 0, "total number of uniformly distributed shards")
 	flag.Parse()

tools/map-shard-for-value/map-shard-for-value.md

@@ -0,0 +1,33 @@
+## Map Shard for Value Tool
+
+### Overview
+
+The `map-shard-for-value` tool maps a given value to a specific shard. This tool helps in determining
+which shard a particular value belongs to, based on the vindex algorithm and shard ranges.
+
+### Features
+
+- Allows specifying the vindex type (e.g., `hash`, `xxhash`).
+- Allows specifying the total number of shards to generate uniformly distributed shard ranges.
+- Designed as a _filter_: Reads input values from `stdin` and outputs the corresponding shard information, so it can be 
+  used to map values from a file or another program.
+
+### Usage
+
+```sh
+make build
+```
+
+```sh
+echo "1\n-1\n99" | ./map-shard-for-value --total_shards=4 --vindex=xxhash
+echo "1\n-1\n99" | ./map-shard-for-value --vindex=hash --shards="-80,80-"
+```
+
+#### Flags
+
+- `--vindex`: Specifies the name of the vindex to use (e.g., `hash`, `xxhash`) (default `xxhash`)
+
+One of these is required:
+- `--shards`: Comma-separated list of shard ranges
+- `--total_shards`: Total number of shards, only if shards are uniformly distributed
+