Docs/README: tweak markup, grammar + misc.

[luzpaz]

Please merge after #5
Preview: https://github.com/luzpaz/getDimensions/blob/DOCS-README/Docs/README.md

[dprojects]

• "within" is more related to time not place?
• "You" - explained before, intended, more gentle.
• "18mm" - is technical variable, "18 mm" is the correct written form?
• indent for images hide them in the section level, it is more clear while reading.
• at this step "you can" not "you will"? if you did the things before you are at the present moment and "you can" not "you will", you will if you do something or in the future, but if you did everything you already there?
• short form like "don't", "you'll" are not recommended to use in written form, you can use it while talking not writing, especially not in documentations.

If You are disappointed or even angry because of the changes just open new pull request, issue, or forum comment. I will try to take it into consideration again. Thanks, for the issues.