Readme: Update development build instructions

[tennox]

For my issue reports I was asked if the error is still happening in develop binary, so I tried to figure out how to build it (as the instructions were outdated).

I managed to compile using the given instructions, so I guess this PR could be merged, but I'm not sure as the built binary is giving me trouble:

brig path not detected

I need to set BRIG_PATH environment variable, otherwise the daemon wont launch, as it only checks for config.yml in current working dir.
I guess there's a TODO related ?

brig repo of latest stable cannot be used

I get error of missing immutable.yml - I guess there is no migration process yet?
So I delete repo and init again, but...

ipfs dir is not detected or set

It again tries to search for IPFS files (config dir I think) in current working dir, but I found I could set it manually in config.yml:
ipfs_path_or_url: "/home/manu/.ipfs"

[sahib]

Hello @tennox,

thanks for your report. Your difficulties seem to arise due to some out dated documentation, since a lot things changed in the develop branch. Some things are not ready for a merge yet, which is a rather unfortunate state. Sadly, I currently have no time to fix all of this (and the other tickets you mentioned) due to time restrictions on my side. Once (and if) I come back to it, I will check this and work towards a merge. Then also this PR will be merged.