Edits to tighten up the documentation

Hi, I've started editing the README to (hopefully) make everything clearer for less advanced users. I only edited the first third or so, up until the Filter example. If you like these changes, I'll finish editing the rest of the doc.

Main changes:

wrote a more succinct description of the library (I used the broad phrase "data analysis and manipulation library," which is how I've seen Pandas described, but this could be narrowed to "dataframe library" or "dataframe API" )
tried to simplify some of the explanations for how things work under the hood (hopefully I stated everything correctly...)
rearranged the instructions in a few places to match the order of operations in the code examples
moved the installation section to the beginning where new users are more likely to see it (I didn't notice it the first time I read the docs)
replaced the instructions for compiling Nim from source with a link to the official nim-lang.org instructions, so non-C developers can see other options (this also keeps the focus on NimData instead of Nim itself)
for each example, added the equivalent syntax in Pandas for users coming from Python
changed code block formatting from 'nimrod' to 'nim' (there was a mix of both, and nimrod doesn't seem to be supported)
tried to standardize terminology a bit (e.g. always "dataframe," never "data frame")

One thing I wasn't sure about is whether "field" and "column" can be used interchangeably, e.g. the procs in the schema example have names like strCol and we'd call these "columns" in Pandas but the docs refer to them as fields. If we can clear this up, I'll add a section that describes DataFrame structure/terminology so everything is consistent.

Hope this helps and thanks for making this library!

bluenote10 / NimData

Edits to tighten up the documentation #67