pgAdmin 4 commit: Improve the README to include comprehensive informati

Improve the README to include comprehensive information on:

- Building the runtime
- Setting up the Python environment
- Configuring the runtime
- Building packages

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=60a0ed61b02bb478acd11c0682f0e044c8d527b0

Modified Files
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README | 256 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 236 insertions(+), 20 deletions(-)