docs(taskfile): add flowchart for intended behaviour of Cache

Mapped out a flowchart to describe the behaviour of taskfile.Cache, so as to aid technical design and implementation.

re go-task#1402

taskfile/README.md

@@ -0,0 +1,43 @@
+# The `taskfile` package
+
+```mermaid
+---
+title: taskfile.Cache behaviour
+---
+flowchart LR
+  %% Beginning state
+    start([A remote Taskfile
+      is required])
+
+  %% Checks to decide
+    cached{Remote Taskfile
+      already cached?}
+
+    subgraph checkTTL [Is the cached Taskfile still inside TTL?]
+    %% Beginning state
+      lastModified(Stat the cached
+        Taskfile and get last
+        modified timestamp)
+
+    %% Check to decide
+      timestampPlusTTL{Timestamp
+        plus TTL is in
+        the future?}
+
+    %% Flowlines
+      lastModified-->timestampPlusTTL
+    end
+
+  %% End states
+    useCached([Use the
+      cached Taskfile])
+    download(["(Re)download the
+      remote Taskfile"])
+
+  %% Flowlines
+    start-->cached
+    cached-- Yes -->lastModified
+    cached-- No -->download
+    timestampPlusTTL-- Yes -->useCached
+    timestampPlusTTL-- No -->download
+```