improve docs

[pameyer]

readme significantly out of date.

[pameyer]

README is slightly less out of date.

[pdurbin]

I'm not sure what the definition of done is for this issue but I guess I'll add some thoughts here as of 8c1e652 especially with an eye toward a developer having DCM spun up to work on IQSS/dataverse#3725

• In the README, see [doc/installation.md](installation instructions). isn't a link.
• We should advise developers to use vagrant up I think. This isn't mentioned in installation.md. Perhaps we should keep installation.md targeted at sysadmins who want to run the DCM for real and create a separate file for developers. A natural choice would be CONTRIBUTING.md, which every GitHub repo should have anyway because you users get a link to it when they go to create an issue.
• After copying secrets.yml.template to secrets.yml "configure the hostlist as appropriate" is not enough information for me to change anything. I think maybe I can leave it as-is for development, but I don't remember.
• You need to have Ansible installed before you run vagrant up.

[pdurbin]

More stuff:

• /var/log/rq_worker.log seems to be an important log file when troubleshooting and should be documented.
• I reopened 403 - Forbidden: allow Dataverse to talk to DCM #10 because I was blocked on getting 403 Forbidden.

[pameyer]

• document /deposit/processed , /deposit/requests (pre/post storage for request files).