Mention Apache's AddDefaultCharset directive in docs - #877

Updates the usage documentation to mention Apache's AddDefaultCharset directive that is included in the default httpd.conf file of some versions of Apache.

Description

This adds a note to the documentation regarding deploying with Apache about looking for the AddDefaultCharset in the httpd.conf and commenting it out if present to prevent issues related to character encoding with replaying archived HTML that uses a different charset.

Motivation and Context

I am suggesting this addition to the documentation because it is an issue I encountered with our production instance of pywb that runs with Apache and uWSGI.

Screenshots (if appropriate):

This is the issue the documentation addition addresses:
pywb_apache_charset_issue

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.