Add documentation to help new users

[louisfelix]

Hi! This PR is a small README documentation contribution. The proposed modifications are the following:

• Indent headers to sub-headers for better reading experience (except the first header)
• Add an "Install" sub-heading to the README (as seen in other exporters: useful hint if a user want to install the exporter but is not familiar with Go)
• Adding "Usage" sub-heading to the README (as seen in other exporters; useful in analysis phases to quickly understand usage without installing and running --help command)
• Change "./Azure-metrics-exporter" to "./azure-metrics-exporter" to reflect default binary name
• Add an "Exporter configuration" sub-heading to re-group corresponding section and inform on the default "azure.yml" (this is not obvious for a new user)
• Add an "Azure account requirements" under "Exporter Configuration" (useful clarification if a user want to configure the exporter but is not familiar with Azure credentials).

Thanks!

[brian-brazil]

Thanks!

[louisfelix]

Thanks to you @brian-brazil for your quick reply!