Update README.rst

[cgestes]

some updates to the README.rst to make it easier to onboard new users.

Some of my collaborators decided to clone the content of the manifest by hand, to avoid using tsrc (because they couldn't make it work) :/

[dmerejkowsky]

OK, but using pip outside a virtualenv is dangerous and may break things down the road :(

What exactly did go wrong with pipx ?

[dmerejkowsky]

I've merged the rest of the changes in the README, which are fine :)

[cgestes]

it seems that a lot of people never heard of pipx, and just install pip. (which is also my case).
So having to install a tool to grab sources is annoying, having to grab two is even more..

I believe almost everyone default to use pip cause it's the standard tool. (especially on windows and mac)

[gdubicki]

I think that pipx should remain the recommended way to install tsrc but we can add a line saying that you can also use plain pip, if you know what you are doing. What do you think, @cgestes?

[Lecrapouille]

@gdubicki @cgestes Remind me my initial remarks #332 I was aware that python has virtual stuff to fix versioning conflict but I never used pipx and pip worked fine :)

[gdubicki]

There's also #343 - haven't seen this issue before today as it was not linked to this PR...

[gdubicki]

Your remaining suggestions about pipx will de facto be applied when #376 will get merged, so I take the liberty to close this PR now.

[gdubicki]

Thank you for your contribution @cgestes! :)