Fix build instructions

[corollari]

Currently the documentation on installing tendermint from source references the now-gone repository wiki, which leads to those links being broken. This PR fixes that by just inlining the content that was in the wiki.

Furthermore, building tendermint with just the instructions provided results in the following error:

Get Certstrap
go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;
	ignoring go.mod;
	see 'go help modules'
go: cannot use path@version syntax in GOPATH mode
make: *** [tools.mk:62: /home/corollari/go/bin/certstrap] Error 1

This PR fixes that by also setting the GO111MODULE environment variable.

[codecov-io]

Codecov Report

Merging #4123 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #4123     +/-   ##
=========================================
+ Coverage   66.57%   66.67%   +0.1%     
=========================================
  Files         247      247             
  Lines       21218    21220      +2     
=========================================
+ Hits        14126    14149     +23     
+ Misses       6036     6019     -17     
+ Partials     1056     1052      -4

Impacted Files	Coverage Δ
consensus/reactor.go	77.1% <0%> (-0.93%)	⬇️
blockchain/v0/reactor.go	78.77% <0%> (-0.76%)	⬇️
consensus/replay.go	72.54% <0%> (+0.78%)	⬆️
p2p/pex/pex_reactor.go	83.86% <0%> (+1.72%)	⬆️
privval/signer_listener_endpoint.go	89.13% <0%> (+2.17%)	⬆️
types/tx.go	91.3% <0%> (+4.34%)	⬆️
privval/signer_server.go	100% <0%> (+4.34%)	⬆️
privval/signer_endpoint.go	83.78% <0%> (+8.1%)	⬆️
blockchain/v2/reactor.go	70.17% <0%> (+19.29%)	⬆️

[tac0turtle]

Thank you for your contribution, we are loving these ❤️

[melekes]

👍