[swift_snapshot_tool] Add a README.

utils/swift_snapshot_tool/README.md

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+
+# Swift Snapshot Tool
+
+Used to list and bisect nightly snapshots off of swift.org.
+
+# Examples
+
+## List snapshots
+
+```
+# xcrun swift run swift_snapshot_tool list
+[INFO] Starting to download snapshot information from github. 
+[INFO] Finished downloading snapshot information from github. 
+0 swift-DEVELOPMENT-SNAPSHOT-2024-09-06-a
+1 swift-DEVELOPMENT-SNAPSHOT-2024-09-05-a
+2 swift-DEVELOPMENT-SNAPSHOT-2024-09-04-a
+3 swift-DEVELOPMENT-SNAPSHOT-2024-08-29-a
+...
+```
+
+The number to the left is just the number back through historical time that the
+value is at. This allows for one to get a sense of the range of time in between
+two bisect numbers since one can look at the range in between them and how time
+varies.
+
+## Bisect
+
+```
+xcrun swift run swift_snapshot_tool bisect --script $SCRIPT_NAME --workspace $DIR_TO_DOWNLOAD_TOOLCHAINS \
+   --good-tag $OLDER_TAG_NAME_THAT_PASSES --bad-tag $NEWER_TAG_NAME_THAT_FAILS
+```
+
+bisect success is defined by `$SCRIPT_NAME` returning 0 as an exit code. All
+other exit codes are considered a failure.
+
+Options:
+
+- workspace: This is the place where we will download toolchains to. Defaults to
+  `/tmp/swift_snapshot_tool_workspace_v1`
+
+- script: This is the script that should be run. We pass in the environment
+  variables `SWIFT_EXEC` and `SWIFT_FRONTEND` to the subscript. If the script
+  returns a zero exit code then the run is considered a succeess. If the script
+  returns a non-zero exit code, then the run is considered a failure.
+
+- good_tag: This is the older tag and is assumed to succeed by returning a zero
+  exit code.
+
+- bad_tag: This is the newer tag and is assumed to fail by returning a non-zero
+  exit code.
+
+- invert: This causes test.sh's result to be inverted. This allows one to bisect
+  backwards from a good state to a bad state. This is useful to determine when
+  an error was fixed.
+
+- branch: This controls the specific branch of snapshots that are downloaded. By
+  default uses development. Also supports the options 5.0 and 6.0.