[MINOR][DOC] Documentation on JVM options for SBT

## What changes were proposed in this pull request?

Documentation and .gitignore

## How was this patch tested?

Manual test that SBT honors the settings in .jvmopts if present

Closes apache#23615 from sadhen/impr/gitignore.

Authored-by: Darcy Shen <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: da-liii

.gitignore

@@ -94,3 +94,6 @@ spark-warehouse/
 *.Rproj.*
 
 .Rproj.user
+
+# For SBT
+.jvmopts

docs/building-spark.md

@@ -138,6 +138,14 @@ To avoid the overhead of launching sbt each time you need to re-compile, you can
 in interactive mode by running `build/sbt`, and then run all build commands at the command
 prompt.
 
+### Setting up SBT's Memory Usage
+Configure the JVM options for SBT in `.jvmopts` at the project root, for example:
+
+    -Xmx2g
+    -XX:ReservedCodeCacheSize=512m
+
+For the meanings of these two options, please carefully read the [Setting up Maven's Memory Usage section](http://spark.apache.org/docs/latest/building-spark.html#setting-up-mavens-memory-usage).
+
 ## Speeding up Compilation
 
 Developers who compile Spark frequently may want to speed up compilation; e.g., by using Zinc