SERVER-108312 Flag Sync getting started README (#39264)

GitOrigin-RevId: d4b90a39d74e376cd260318343f7717ab2425a67

tools/flag_sync/README.md

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+# What is Flag Sync?
+
+Flag Sync is a tool to allow engineers to quickly introduce or disable flags on remote client systems.
+Currently, this is used to enable or disable certain Bazel flags across user workstations or CI hosts.
+
+# Basic usage
+
+## Prerequisites
+
+Make sure you have valid AWS admin credentials for `devprod-build`. `aws configure sso` is the easiest way to do so.
+
+## Get list of flags for a namespace
+
+`bazel run //tools/flag_sync:client flag get "{namespace}"`
+
+## Create a flag
+
+`bazel run //tools/flag_sync:client flag create "{namespace}" "{flag_name}" "{flag_value}"`
+
+## Toggle flag on/off
+
+`bazel run //tools/flag_sync:client flag toggle-off "{namespace}" "{flag_name}"`
+`bazel run //tools/flag_sync:client flag toggle-on "{namespace}" "{flag_name}"`
+
+## Delete a flag
+
+`bazel run //tools/flag_sync:client flag delete "{namespace}" "{flag_name}"`
+
+## Namespaces
+
+- `user-prod` - Namespace for flags to be synced with user workstations.
+- `ci-prod` - Namespace for flags to be synced with CI hosts.
+
+## Example
+
+### Enable --config=local for users.
+
+Create a new config local flag under the `user-prod` namespace:
+`bazel run //tools/flag_sync:client flag create "user-prod" "local" "common --config=local"`
+
+If the flag exists already, you can simple run:
+`bazel run //tools/flag_sync:client flag toggle-on "user-prod" "local"`