Describe when and why to use init: false

Add some description to the "advanced options" documentation for the `IPFS` constructor detailing that `init: false` should really be used if a repo has already been initialized *by IPFS* and that simply calling `repoInstance.init()` doesn't init a repo with everything IPFS needs.

This partially handles #1245.

README.md

@@ -194,7 +194,11 @@ const repo = <IPFS Repo instance or repo path>
 const node = new IPFS({
   repo: repo,
   init: true, // default
-  // init: false,
+  // init: false, // If you have already initialized the repo via `new IPFS()`.
+  //              // Be careful using this on an IPFSRepo instance you have
+  //              // initialized yourself -- the IPFS constructor initializes
+  //              // repos with extra options, so calling `init()` on an
+  //              // IPFSRepo may not always be compatible with IPFS.
   // init: {
   //   bits: 1024 // size of the RSA key generated
   // },