add ToolCache docs

Author: Colin Wahl

docs/05-Tool-Cache.md

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+# GitHub.Actions.ToolCache
+
+> Functions necessary for downloading and caching tools.
+
+This module exposes bindings to the [@actions/tool-cache package](https://github.com/actions/toolkit/tree/main/packages/tool-cache).
+
+## Usage
+
+#### Download
+
+You can use this to download tools (or other files) from a download URL:
+
+```purescript
+
+node12Path <- ToolCache.downloadTool "https://nodejs.org/dist/v12.7.0/node-v12.7.0-linux-x64.tar.gz"
+```
+
+#### Extract
+
+These can then be extracted in platform specific ways:
+
+```purescript
+const tc = require('@actions/tool-cache');
+
+node12ExtractedFolder <- case platform of
+  Just Win32 -> do
+    node12Path <- ToolCache.downloadTool' "https://nodejs.org/dist/v12.7.0/node-v12.7.0-win-x64.zip"
+    ToolCache.extractZip' node12Path
+  Just Darwin -> do
+    node12Path <- ToolCache.downloadTool' "https://nodejs.org/dist/v12.7.0/node-v12.7.0.pkg"
+    ToolCache.extractXar' node12Path
+  _ -> do
+    node12Path <- ToolCache.downloadTool' "https://nodejs.org/dist/v12.7.0/node-v12.7.0-linux-x64.tar.gz"
+    ToolCache.extractTar node12Path
+```
+
+#### Cache
+
+Finally, you can cache these directories in our tool-cache. This is useful if you want to switch back and forth between versions of a tool, or save a tool between runs for self-hosted runners.
+
+You'll often want to add it to the path as part of this step:
+
+```purescript
+const tc = require('@actions/tool-cache');
+const core = require('@actions/core');
+
+example = do
+  node12Path <- ToolCache.downloadTool' "https://nodejs.org/dist/v12.7.0/node-v12.7.0-linux-x64.tar.gz"
+  node12ExtractedFolder <- ToolCache.extractTar node12Path
+  cachedPath <- ToolCache.cacheDir' { sourceDir: node12ExtractedFolder, tool: "node", version: "12.7.0" }
+  liftEffect $ do
+    Core.addPath cachedPath
+```
+
+You can also cache files for reuse.
+
+```purescript
+ToolTache.cachefile' { sourceFile: "path/to/exe", targetFile: "destFileName.exe", tool: "myExeName", version: "1.1.0"}
+```
+
+#### Find
+
+Finally, you can find directories and files you've previously cached:
+
+```purescript
+nodeDirectory <- ToolCache.find' { toolName: "node", versionSpec "12.x" }
+liftEffect $ Core.addPath nodeDirectory
+```
+
+You can even find all cached versions of a tool:
+
+```purescript
+allNodeVersion <- ToolCache.findAllVersions "node"
+```