README: update manual install instructions

[chriscool]

The manual install instructions haven't worked since since #90. Let's update them.

This is an alternative to #100.

[rafasc]

The markdown links are broken.

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Adding sharness as a submodule, having a header on each test.t as follows:

: "${SHARNESS_TEST_DIRECTORY:=.}"
: "${SHARNESS_TEST_SRCDIR:=../sharness}"
. "${SHARNESS_TEST_SRCDIR}/sharness.sh"

and aggregate-results.sh and Makefile can be either copied or symbolic linked, mostly works;

However, it doesn't play well extensions, because they're loaded from $SHARNESS_TEST_SRCDIR: https://github.com/chriscool/sharness/blob/b57a2a49d6b5663388c8a146d7af5fdf63491fc9/sharness.sh#L488

Wouldn't it make more sense to allow those extensions to be tracked alongside the parent project? For that we would need to either load it from $SHARNESS_TEST_DIRECTORY or add another env to specify a custom directory. This would allow users to keep the submodule clean without custom changes.

Or am I missing the usecase for sharness.d ?

[chriscool]

@rafasc yeah, you are right that allowing extensions to be tracked alongside the parent project could be a good idea for some projects. Other projects might want to reuse extensions developed elsewhere though.

So I think I should merge this PR as is. But then I am open to improvements to this doc about this, so I have opened #103

About markdown links, I am going to take a look.

Thank you for your comment!

[chriscool]

I fixed the links in: 0a040a8
Sorry about forgetting it.