Add a documentation for warmed containers configurations.

Author: style95

docs/warmed-containers.md

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+# Warmed Containers
+
+Warmed containers can improve their performance by skipping the container initialization step.
+It may be beneficial to configure the number of warmed containers to keep, and the duration to keep them according to the characteristics of workloads.
+
+## Configuration
+
+The configurations are only effective with the FPC scheduler.
+They can be configured using the limit configurations for each namespace.
+
+```scala
+case class UserLimits(invocationsPerMinute: Option[Int] = None,
+                      concurrentInvocations: Option[Int] = None,
+                      firesPerMinute: Option[Int] = None,
+                      allowedKinds: Option[Set[String]] = None,
+                      storeActivations: Option[Boolean] = None,
+                      warmedContainerKeepingCount: Option[Int] = None,
+                      warmedContainerKeepingTimeout: Option[String] = None)
+```
+
+So those can be configured in the same way that operators configure the `invocationPerMinute` limit.
+
+```json
+{
+  "_id": "guest/limits",
+  "invocationPerMinutes": 10,
+  "warmedContainerKeepingCount": 8,
+  "warmedContainerKeepingTimeout": "24 hours"
+}
+```
+
+The namespace-specific configurations would override the default, system-wide configurations.
+In the above example, the system will keep 8 warmed containers for 24 hours even if there is no activation at all.