Fix link to install guide
Merge pull request #60 from Metaswitch/fix_docs_link

[Reviewer: Ellie] Fix link to install guide
Allow arbitrary numbers to be created by users with the NGV-API-Key
Allow reuse of the same specific ID
Keep types consistent
Merge pull request #61 from Metaswitch/arbitrary_numbers_api

[Reviewer: Ellie] Allow arbitrary numbers to be created by users with the NGV-API-Key