Add manual build instructions

[blegat]

Add instructions on how to link to a manual installation of the library.
It may help with #5 which is still an issue for ArchLinux and the method mentioned in the ArchLinux wiki is incorrect as detailed in #5 (comment)

[coveralls]

Pull Request Test Coverage Report for Build 121

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.357%

---

Totals
Change from base Build 119:	0.0%
Covered Lines:	183
Relevant Lines:	280

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 122

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+19.8%) to 85.116%

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Files with Coverage Reduction	New Missed Lines	%
src/Arpack.jl	1	81.61%

Totals
Change from base Build 119:	19.8%
Covered Lines:	183
Relevant Lines:	215

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #69 into master will increase coverage by 19.75%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #69       +/-   ##
===========================================
+ Coverage   65.35%   85.11%   +19.75%     
===========================================
  Files           2        2               
  Lines         280      215       -65     
===========================================
  Hits          183      183               
+ Misses         97       32       -65

Impacted Files	Coverage Δ
src/libarpack.jl	87.5% <0%> (+6.92%)	⬆️
src/Arpack.jl	81.6% <0%> (+31.25%)	⬆️

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[andreasnoack]

Thanks. It would be great if you could expand it to general installation instructions explaining that for most users, it should be fine just to add Arpack or something like that?

[blegat]

@andreasnoack I have added general instructions