Fix markdown table and add more descriptions to README.md

Author: akubera

README.md

@@ -12,12 +12,18 @@
 
 
 
-Arbitary-precision decimal numbers implemented in pure Rust.
+Arbitrary-precision decimal numbers implemented in pure Rust.
 
 ##  Community
 
 Join the conversation on Zulip: https://bigdecimal-rs.zulipchat.com
 
+Please share important stuff like use-cases, issues, benchmarks, and
+naming-convention preferences.
+
+This project is currently being re-written, so if performance or flexibility
+is lacking, check again soon and it may be fixed.
+
 ## Usage
 
 Add bigdecimal as a dependency to your `Cargo.toml` file:
@@ -47,15 +53,21 @@ sqrt(2) = 1.41421356237309504880168872420969807856967187537694807317667973799073
 
 ### Compile-Time Configuration
 
-You can set a few default parameters at compile-time via environment variables.
+You can set a few default parameters at _compile-time_ via environment variables:
 
-+-------------------------------------------------+------------+
 |  Environment Variable                           | Default    |
-+-------------------------------------------------+------------+
+|-------------------------------------------------|------------|
 |  `RUST_BIGDECIMAL_DEFAULT_PRECISION`            |  100       |
 |  `RUST_BIGDECIMAL_DEFAULT_ROUNDING_MODE`        | `HalfEven` |
 |  `RUST_BIGDECIMAL_EXPONENTIAL_FORMAT_THRESHOLD` |  5         |
-+-------------------------------------------------+------------+
+
+These allow setting the default [Context] fields globally without incurring a runtime lookup,
+or having to pass Context parameters through all calculations.
+(If you want runtime control over these fields, you will have to pass Contexts to your functions.)
+Examine [build.rs] for how those are converted to constants in the code (if interested).
+
+[Context]: https://docs.rs/bigdecimal/latest/bigdecimal/struct.Context.html
+[build.rs]: ./build.rs
 
 
 #### Default precision
@@ -65,14 +77,14 @@ The default value of this variable is 100.
 
 This will be used as maximum precision for operations which may produce infinite digits (inverse, sqrt, ...).
 
-Note that other operations, such as multiplication, will preserve all digits, so multiplying two 70 digit numbers
-will result in one 140 digit number.
-The user will have to manually trim the number of digits after calculations to reasonable amounts using the
-`x.with_prec(30)` method.
-
-A new set of methods with explicit precision and rounding modes is being worked on, but even after those
-are introduced the default precision will have to be used as the implicit value.
+Note that other operations, such as multiplication, will preserve all digits;
+so multiplying two 70 digit numbers will result in one 140 digit number.
+The user will have to manually trim the number of digits after calculations to
+reasonable amounts using the `x.with_prec(30)` method.
 
+A new set of methods with explicit precision and rounding modes is being worked
+on, but even after those are introduced the default precision will have to be
+used as the implicit value.
 
 #### Rounding mode
 
@@ -132,12 +144,6 @@ $ cargo run
 > This is for flexibility and performance.
 
 
-## Improvements
-
-Work is being done on this codebase again and there are many features
-and improvements on the way.
-
-
 ## About
 
 This repository contains code originally meant for a bigdecimal module