readme: Update installation instructions

[Bjwebb]

#650

[simon-20]

I can now get all the steps to run, and the webserver starts up without problems.

I've so far noticed two problems:

1. The images don't display. For example, the path for the graph of total activities in the HTML is /generated/img/aggregate/activities.png. On disk, the path (relative to the root of the git repo) is ./out/img/aggregate/activities.png

2. The JSON links are broken. For instance, when trying to access the following URL:

http://localhost:8000/stats/gitaggregate-dated/unique_identifiers.json

This is stored on disk, relative to the root of git repo in,

./stats-calculated/gitaggregate-dated/

But the HTTP request is trying to access that file one folder up, i.e., it's trying to access:

/home/my-user/dev/stats-calculated/gitaggregate-dated/unique_identifiers.json

when it should be accessing:

/home/my-user/dev/IATI-Dashboard/stats-calculated/gitaggregate-dated/unique_identifiers.json

[Bjwebb]

@simon-20 This should now work as expected.