The existing documentation of http.Dir is clear in that, if you want to hide
your files and directories that start with a period, you must create
a custom FileSystem. However, there are currently no example on how
to create a custom FileSystem. This commit provides an example.

Fixes #20759

Change-Id: I5a350675536f81412af384d1a316fd6cd6241563
GitHub-Last-Rev: 8b0b644cd02c59fe2461908304c44d64e8be431e
GitHub-Pull-Request: golang/go#26768
Reviewed-on: https://go-review.googlesource.com/127576Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>