Add documentation about the TMPDIR environment variable for better discovery

Author: cmcgee1024

Documentation/SwiftlyDocs.docc/automated-install.md

@@ -75,6 +75,7 @@ If you want to install swiftly, or the binaries that it manages into different l
 
 - term `SWIFTLY_HOME_DIR`: The location of the swiftly configuration files, and environment scripts
 - term `SWIFTLY_BIN_DIR`: The location of the swiftly binary and toolchain symbolic links (for example swift, swiftc, and so on)
+- term `TMPDIR`: The temporary directory to be used by swiftly for temporary files, such as downloads that can be very large until they are cleaned up.
 
 Sometimes swiftly can't automatically detect the system platform, or isn't supported by swift.
 You can provide the platform as an option to the `swiftly init` subcommand:

Documentation/SwiftlyDocs.docc/swiftly-cli-reference.md

@@ -53,10 +53,14 @@ Likewise, the latest snapshot associated with a given development branch can be
     $ swiftly install 5.7-snapshot
     $ swiftly install main-snapshot
 
- Install whatever toolchain is currently selected, such as the the one in the .swift-version file:
+Install whatever toolchain is currently selected, such as the the one in the .swift-version file:
 
     $ swiftly install
 
+NOTE: Toolchains are downloaded to a temporary file that is later cleaned up by the installation process. If these files are too big for your usual temporary directory you can choose another location by setting the `TMPDIR` environment variable.
+
+    $ TMPDIR=/large/file/tmp/storage swiftly install latest
+
 
 **--use:**
 

Sources/Swiftly/Install.swift

@@ -40,9 +40,15 @@ struct Install: SwiftlyCommand {
             $ swiftly install 5.7-snapshot
             $ swiftly install main-snapshot
 
-         Install whatever toolchain is currently selected, such as the the one in the .swift-version file:
+        Install whatever toolchain is currently selected, such as the the one in the .swift-version file:
 
             $ swiftly install
+
+        NOTE: Toolchains are downloaded to a temporary file that is later cleaned up by the installation \
+        process. If these files are too big for your usual temporary directory you can choose another location \
+        by setting the `TMPDIR` environment variable.
+
+            $ TMPDIR=/large/file/tmp/storage swiftly install latest
         """
     ))
     var version: String?