Rephrase instructions for local usage (#327)

The rewords some of the instructions in the README for local coverage measurement. The major changes & their motivations are:

- indicate the purpose of each step before describing the mechanics of the step. This also makes it easier to indicate that one does not necessarily need to measure summary statistics if one's only goal is to find lines lacking coverage.
- prioritize `Pkg.test(; coverage=true)` above manual measurement. Most users will want the simpler option.
- clarify the role of the `.info` tracefile. This allows users to determine when they need to supply the more complex set of options. By presenting it as a choice it also makes it clearer that you don't need to execute both lines.
- acknowledge limitations in the accuracy of Julia's coverage analysis

Author: timholy

README.md

@@ -22,22 +22,25 @@ Most users will want to use [Coverage.jl](https://github.com/JuliaCI/Coverage.jl
 
 ### Code coverage
 
-*Step 1:* Navigate to your test directory, and run julia with the `--code-coverage` option:
+*Step 1: collect coverage data.* If you are using your default test suite, you can collect coverage data with `Pkg.test("MyPkg"; coverage=true)`. Alternatively, you can collect coverage data manually: in the terminal, navigate to whatever directory you want to start from as the working directory, and run julia with the `--code-coverage` option:
+
 ```sh
 julia --code-coverage=user
+```
+or more comprehensively (if you're interested in getting coverage for Julia's standard libraries)
+```sh
 julia --code-coverage=tracefile-%p.info --code-coverage=user  # available in Julia v1.1+
 ```
+You can add other options (e.g., `--project`) as needed. After the REPL starts, execute whatever commands you wish, and then quit Julia. Coverage data are written to files when Julia exits.
 
-*Step 2:* Run your tests (e.g., `include("runtests.jl")`) and quit Julia. (If you use `Pkg.test` to run your tests, set the `coverage` keyword argument to `true`, i.e. `Pkg.test("MyPkg"; coverage=true)`.)
-
-*Step 3:* Navigate to the top-level directory of your package, restart Julia (with no special flags) and analyze your code coverage:
+*Step 2: collect summary statistics (optional).* Navigate to the top-level directory of your package, restart Julia (with no special flags) and analyze your code coverage:
 
 ```julia
 using Coverage
 # process '*.cov' files
 coverage = process_folder() # defaults to src/; alternatively, supply the folder name as argument
-coverage = append!(coverage, process_folder("deps"))
-# process '*.info' files
+coverage = append!(coverage, process_folder("deps"))  # useful if you want to analyze more than just src/
+# process '*.info' files, if you collected them
 coverage = merge_coverage_counts(coverage, filter!(
     let prefixes = (joinpath(pwd(), "src", ""),
                     joinpath(pwd(), "deps", ""))
@@ -51,10 +54,15 @@ covered_lines, total_lines = get_summary(coverage)
 ```
 The fraction of total coverage is equal to `covered_lines/total_lines`.
 
-To discover which functions lack testing, browse through the `*.cov` files in your `src/`
-directory and look for lines starting with `-` or `0` - those lines were never executed.
+*Step 3: identify uncovered lines (optional).* To discover which functions lack testing, browse through the `*.cov` files in your `src/`
+directory and look for lines starting with `-` or `0`, which mark lines that were never executed.
 Numbers larger than 0 are counts of the number of times the respective line was executed.
+Note that blank lines, comments, lines with `end` statements, etc. are marked with `-` but do not count against your coverage.
+
+Be aware of a few limitations:
 
+- a line that can take one of two branches gets marked as covered even if only one branch is tested
+- currently, code run by Julia's internal interpreter [is not marked as covered](https://github.com/JuliaLang/julia/issues/37059).
 
 ### Memory allocation