Update readme.md

[MarGraz]

When you running your solution on Azure Functions, you didn't have a storage, the user must use the cache in RamDirectory. This info could be useful, because otherwise the user will struggle with error: "Temp folder is not found". Thank you

[pbrown-smsc]

Yes, this would be a good thing. I did not realize that RAMDirectory really must used if you build the index say, in an AzureFunction, and then search it from another Azure App Service