fix broken links to MAIN.md

[awesome]

$ ag -l MAIN.md | tee files_list.txt
_ci/publish-amis-in-new-account.md
examples/consul-examples-helper/README.md
examples/consul-ami/README.md
modules/consul-cluster/README.md
modules/install-consul/README.md
modules/consul-iam-policies/README.md
modules/run-consul/README.md
modules/consul-client-security-group-rules/README.md
modules/consul-security-group-rules/README.md
$ while read file; do sed -E -i '' -e 's/MAIN\.md/modules\/consul-cluster/g' -e 's!(\[consul-cluster) (module|example)!\1!g' "$file"; done < files_list.txt
$ rm files_list.txt

[awesome]

1) added README for /modules and /examples with info from @brikis98: https://github.com/hashicorp/terraform-aws-consul/pull/79/files/079e75015a5d89e7ffc89997aa0904e9de4cdb97#r212763365 2) edited files by hand 3) used script on remaining files 4) git merge -s recursive -X theirs fix_doco_links_to_main_md_2 to maintain original pull request https://github.com/hashicorp/terraform-aws-consul/pull/79 --LMK if you want to close this PR and for me to submit another PR with branch fix_doco_links_to_main_md_2 (which only exists on my laptop)

$ ag MAIN.md
examples/consul-examples-helper/README.md
4:[consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md). After running `terraform apply` on the example, if you run 

examples/consul-ami/README.md
15:in Consul). To see how to deploy this AMI, check out the [consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md). 
41:[consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md).

modules/consul-cluster/README.md
58:Check out the [consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md) for fully-working sample code.
69:subnets and/or your default VPC (as in the [consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md)), which is OK for testing
73:servers by using AWS tags. If you're running the [consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md), the

modules/consul-iam-policies/README.md
47:Check out the [consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md) for working sample code.

modules/run-consul/README.md
47:See the [consul-cluster example](https://github.com/hashicorp/terraform-aws-consul/tree/master/MAIN.md) for fully-working sample code.
$ ag -l MAIN.md | tee files_list.txt
examples/consul-examples-helper/README.md
examples/consul-ami/README.md
modules/consul-cluster/README.md
modules/consul-iam-policies/README.md
modules/run-consul/README.md
$ while read file; do sed -i '' -e 's/MAIN\.md/modules\/consul-cluster/g' -e 's!MAIN\.md!examples/root-example!g' "$file"; done < files_list.txt
$ rm files_list.txt 

[awesome]

fixed example links